Sound in Team Fortress 2 got really messed up after yesterday's update, most noticeable with the demoman launcher and the soldier's rocket launcher.

The solution is simple:

snd_noextraupdate 1

and if that doesn't work also:

snd_mix_minframetime 0

These get overriden by the game at map change, so it's best to put them in autoexec.cfg or bind a key to them.

VALVe confirmed this issue, and this is a temporary fix until it will be fixed in engine.