Doesn't say anything about royalty free or original music / sounds or anything really in the rules that I saw.
Regardless of the rules I enjoy the challenge of creating my own assets and am going with unique everything this time or bust -- Here's my 0.02 on the whole thing:
I think using assets as placeholders of sorts in order to get your project done, and even going live with them (provided that you own them or they are public domain and credit is given / you don't try to pass it off as your own work) is fair game.
Certainly there is more to be said for fully originally assets, so I think launching with completely original everything would get you more praise and points -- on the other hand sometimes you have to make due.
I just did GGAJam and had to settle for a public domain soundtrack that I found on a forum and I wasn't feeling too creative so I went with a tribute of sorts to an existing game. I pointed out the similarity of my game, and for the music gave credit to the original composer and the user who put together this particular version and uploaded it. If people cared enough about the game then I would go back and added my own version of the public domain song used, or if i really believed in the game would have launched with something fully original.
I think I made the right choice in that last case with a pre-existing soundtrack because my game wasn't good / interesting enough to most people to lose a few extra hours of sleep to make an original soundtrack -- and then again, perhaps that would have made it more special, so I think I see a balance in all of that.
I think at the end of the day its the games that are the most interesting and fun that will capture the player's attention more than original assets (sometimes).
Again though - as for the rules on this one I didn't see one.