Fix: Unity Timeline Audio Clip Not Playing

Here is how to fix Unity Timeline audio clips not playing. Your cutscene plays, animations trigger correctly, but the audio track produces complete silence. The Timeline editor shows the audio waveform on the clip, scrubbing in preview mode plays audio fine, but at runtime — nothing. The AudioClip is assigned, the track is not muted in the editor, and yet the game is silent where it should not be.

The Symptom

A PlayableDirector plays a Timeline asset that contains one or more Audio Tracks. In the editor preview (scrubbing the timeline), audio plays correctly. When you enter Play mode or run a build, the audio track is completely silent while all other tracks (Animation, Activation, Signal) work normally.

No errors appear in the console. The PlayableDirector state shows Playing. The Timeline is clearly running because other tracks execute their clips on schedule.

What Causes This

Missing AudioSource binding. Every Audio Track in a Timeline requires a bound AudioSource component. The PlayableDirector’s bindings panel maps each track to a scene object. If the Audio Track binding is None, audio has no output target. The editor preview bypasses this by using an internal preview AudioSource, which is why scrubbing works but runtime does not.

Null clip asset reference. If the AudioPlayableAsset on the clip lost its reference to the AudioClip file (due to a moved or deleted asset), the clip plays silence. The waveform preview may still show from cached data even though the actual reference is broken.

Volume curve set to zero. Each audio clip in Timeline has a volume curve property. If this curve was accidentally set to a constant value of 0 (or an animation curve that evaluates to 0 at the current time), the audio plays at zero volume.

Mute or solo state persisted. The Timeline editor has Mute (M) and Solo (S) buttons per track. These states are serialized into the Timeline asset. If you muted a track during editing and forgot to unmute it, the track stays muted at runtime.

The Fix

Step 1: Bind an AudioSource to the track. Select the GameObject with the PlayableDirector. In the Inspector under Bindings, find the Audio Track entry. Assign an AudioSource component from the scene.

// Bind AudioSource via script if setting up at runtime
var director = GetComponent<PlayableDirector>();
var timeline = director.playableAsset as TimelineAsset;

foreach (var track in timeline.GetOutputTracks())
{
    if (track is AudioTrack audioTrack)
    {
        director.SetGenericBinding(audioTrack, audioSource);
    }
}

Step 2: Verify clip asset references. Click each audio clip in the Timeline. In the Inspector, check that the AudioPlayableAsset has a valid Clip field pointing to an actual AudioClip asset. If it says “Missing” or “None,” re-assign the audio file.

Step 3: Check the volume curve. Select the audio clip in Timeline. In the Inspector, expand the clip properties. The volume property should show a curve or a constant value of 1. If it is 0, reset it.

// Debug: log all audio clip volumes in a timeline
foreach (var track in timeline.GetOutputTracks())
{
    if (track is AudioTrack audioTrack)
    {
        foreach (var clip in audioTrack.GetClips())
        {
            var asset = clip.asset as AudioPlayableAsset;
            Debug.Log($"{clip.displayName}: clip={asset.clip}, vol curve keys={clip.curves}");
        }
    }
}

Step 4: Unmute the track. Open the Timeline window, look for the M (mute) icon on the audio track. If it is highlighted, click it to unmute. Also check that no other track has Solo (S) enabled, as Solo mutes all non-soloed tracks.

Build-Specific Issues

If audio works in the editor but not in a build, verify the AudioClip is actually included. Timeline assets reference clips indirectly through AudioPlayableAsset sub-assets. If the clip is not otherwise referenced by any scene object and is not in a Resources or Addressables group, the build system may exclude it. Add the clip to a Resources folder or create an explicit reference in a MonoBehaviour.

Understanding the issue

Audio mixing is multiplicative. A bug in one channel's gain affects everything downstream. Bugs that cascade through the mix are hardest to diagnose.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

This bug class disproportionately affects late-stage development. The work to surface it is interactive testing in realistic conditions, which only really happens after the gameplay is in place and assets are populated. Catching it early requires deliberate testing of conditions that look unimportant.

At the engine level, the behavior comes from a deliberate design decision in Unity. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

For shipping games, the safest verification is a staged rollout. Apply the fix to 1% of players for 24 hours; watch the affected metric; expand if green. Skipping the staged rollout means the verification is the entire player base, which is too high a stakes for most fixes.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

Live games surface this bug class at scale. What's a rare edge case in development becomes a daily occurrence once you have a few thousand concurrent players. The class isn't 'this player has a unique setup'; it's 'one in N thousand sessions will trigger this exact combination'.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

Performance implications matter when this bug class scales with player count or asset count. A bug that fires once per session is annoying; a bug that fires once per frame compounds. After fixing, profile the affected code path under realistic load. The fix that's correct for one entity may be too slow for ten thousand.

Diagnostic approach

The diagnostic tools available depend on your engine and platform. Use the engine's native profilers and debug overlays before reaching for external tools. The native tools have context that external tools lack - they know which subsystem owns the code, which assets are loaded, and what state the engine is in.

For Unity-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Third-party plugins often provide better diagnostics for their own behavior than the engine does. If the affected code is in a plugin, check the plugin's documentation for debug modes, verbose logging, or inspector tools - these can save hours of investigation when they exist.

Within Unity, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“Timeline preview lies to you — it uses its own AudioSource. Runtime requires you to provide one.”

Common Gotcha: Spatial Blend

If the bound AudioSource has Spatial Blend set to 1 (fully 3D) and the AudioSource is far from the Audio Listener, the audio plays but is inaudible due to distance attenuation. Set Spatial Blend to 0 for non-positional Timeline audio like music or voiceover.

Nine times out of ten it is a missing binding. The editor preview hides this from you because it creates its own temporary AudioSource.