Rules come from another board, it's been a long time but I think the site might have had something to do with a racing game, maybe Trackmania? Anyway I thought it was a good trio of rules.... it might not be verbatim though.
1. Musicians can post any track or tracks of theirs that they want, but before they can they've got to leave a comment on a song that was posted previously. You're not limited to one song per post, but a musician making a post only has to comment on one song so keep that in mind. So a post where someone adds one of their songs should go something like: "Hey, I liked the vocal effects on the song you posted, Kitsune. I think you could improve the chord progression by adding a seventh to that D-minor chord on the electric triangle. Please don't turn me into an animal. Here's an instrumental song I recorded on harmonica and xylophone. It's about how all airline pilots are secretly spies for the lizard-people."
That middle sentence is optional, but encouraged.
It's not required that a song-poster specifically comment on the last song posted, but it is encouraged. Try to be considerate and avoid keeping others in too much suspense about their work.
2. Keep comments positive and constructive -- that doesn't mean you can't criticize, but remember anybody who spends time writing their songs pours their heart and soul into writing it, and years of practice into the craft. Try to find something nice to say along with your criticisms, or, if you really can't do that, at least try to be helpful. And protip: if you have an outright statement along the lines of "this comment is helpful", or, god-forbid, have to explain why it helps, then it's not actually helpful.
I've seen no sign of any real, malicious trolling since I've signed up here, so I really don't think this one is going to be a problem.
3. Having to post a comment in order to post a song doesn't mean you have to post a song in order to post a comment. Everybody is welcome here! Just so long as they're willing to put in the effort to be nice, of course, but like I said everyone I've talked to here so far has shown that already. So don't be shy, I don't bite (well, except literally)!
4. Remember rule six!
Well, someone has to start us off and, since I'm right here, I'm gonna make a second post for it just to keep this whole thing organized. Seriously hoping I'm not the only musician here now.