The midi files only work in 64-bit builds of the game. There's no reason why you can't hear the music. Please send a video detailing what happened. I haven't added in a music volume setting, but I have added in a volume setting which indeed has a tendency to set itself to 0 sometimes. However, this only affects sounds that are NOT midi files.
So this isn't a bug everyone has, you're like the only person who is telling me this. And that's not a good thing.