Quick answer: Pygame.mixer.music using a dedicated channel; high-priority sounds not preempting it? Music isn't on a normal channel; preemption logic differs.
UI click sound should duck music; music keeps playing.
Manage music as Sound
Load music as Sound; play on a reserved channel. Standard preemption rules apply.
Or duck via volume
Music volume lowered when UI plays; restored after. Less precise; works.
Audit music vs sound
Each audio asset: which channel system. Pick deliberately.
“Music channel is separate from Sound channels. Different rules.”
If your audio has ducking, decide whether to use the music channel. Each path has trade-offs.