Yeah, same happened with us originally, then we had another friend wanting to jump in with us and the music mod had issues for them. So we just played without it. But, going all the way through Moon Lord and what not and having the Calamity bosses being silent fights... Eh? I really dig the music they got for the mod So ya, we had to all manually download it from here and drag & drop into the mod folder for it to work properly.i did that already the bug simply worn off after a while, go figure
I have seen Steam forum posts and Reddit posts saying the same thing, most just say to go get it from the Calamity Discord. So I guess it's just the the in game Mod Browser one that's somehow bugged to work or not work at a coin flip