Solved: The unplayable music was .mp3, which Lix doesn't support
From IRC:
Flopsy: SimonN: question about music compatibility in Lix, I'm guessing there are some file types which are not compatible like .ogg?
Flopsy: Just recorded a session and about 80% of my music wasn't playing :(
Ogg should play perfecly.
What is the exact issue: In some levels, Lix played no track at all? Or Lix played a track in every level, but the track selection was a few tracks over and over?
Check data/log.txt near the end for complaints about unplayable music files: Ctrl+F for "unplayable music" without quotes.
If data/log.txt lacks any entries about unplayable music, you might have had bad luck with the random track selection. Lix might play the same track in every level, omitting all others. Maybe the purely random track selection is bad, and it should be a random permutation that doesn't repeat unless all tracks have played once.
-- Simon