Playing with three friends tonight and the music and most other sound effects stopped for three of us, the original host was not having this issue. You could hear voices and gunfire but no music, no sound from enemies, no sound when opening boxes, etc. This got to be annoying so we quit and I started a new game, I hosted and used a different character but the problem was still there. The original host joined my game also with a different character and continued to be able to hear music and other sound
I also have this issue every time I play co-op and I’m not the one hosting. It seems to happen when the frame rate drops. The frame rate recovers but the sound doesn’t come back until I Fast Travel.