Never thought I would see coop tetris, it's a cool idea! I think it would be great with music and sounds.
Viewing post in 2gether Tetris jam comments
For sound effects, I can recommend sfxr, it's very fast and easy to use (but the sounds are very generic).
However, I believe it's accepted to use free assets in a game jam (probably better to ask the organizers about this). I can recommend freesound for... sounds, and Kevin MacLeod for music.
Hope that helped!