I was playing multiplayer with one other person, and in one session we completed the first Act. After completing it, we both quit, and turned off the game for several hours. Later that day, we returned to Hammerwatch, other person (who was the host) loaded the auto-save, and I right clicked his steam profile and clicked join game to re-join our multiplayer game session. When I loaded into the level, the title screen music was still playing for me (not the act 2 music/floor 4 [or w/e floor it is] music, the other person playing with me had the proper music playing). I tried quitting the game session (without quitting Hammerwatch) and joining via the multiplayer -> join game feature/function. No dice. I tried quitting Hammerwatch entirely and re-joining. No dice. We tried both restarting. The other person left the game, and re-hosted it, then I joined. Now BOTH of us were hearing the title screen music still. Dying and respawning, as well as switching floors had no effect. I have, as of yet, found no way to fix this short of starting an entirely new game session from the beginning.
P.S. this was in Adventure mode.