This bug affects the Abyssal Monument, The Stronghold and End Wilds levels. There is only one music missing in each of these three levels. The missing songs are:
-Abyssal Monument: known on spotify as "Pit Fight" from the music album Minecraft Dungeons: Hidden Depths.
-The Stronghold: known on spotify as "Closer" from the music album Minecraft Dungeons: Echoing Void.
-End Wilds: known on spotify as "Void" from the music album Minecraft Dungeons: Echoing Void.
How to reproduce the bug
You just have to go to these 3 levels and complete all the available ambushes of the levels so that you realize that these songs are not in the game.
By the way, an ambush is the event that summons enemies in an area and during that event another different song begins to play according to the level.
Where to listen to the missing music?
Here I leave the three original soundtracks missing from the game: