OK, this game is pretty simple. Basically, just pick a theme song for the person who posts above you and share it via grooveshark. Give a reason if you can. It doesn't need to embody everything about the person's personality, but can also relate just to his/her avatar, whatever... Be playful as well - nobody should take your choice too seriously. Then the next poster will do the same thing, choosing a theme song for you.
Again, it doesn't have to just relate to the avatar, but better to relate to the personality if you know the poster well enough.