I think the idea is to share, but no, you don't need to share everything. These Jams are just for fun anyway :)

If you want to be helpful, you could include comments in the code highlighting areas which won't work due to music/sprites being withheld, so if anyone decides to work on the game they know what they need to replace.

I think one of the key objectives of the jam is to get developers to expose themselves to GitHub. I've got a Github account for the first time ever thanks to this jam, and I uploaded my source code to it. I think the source code is pretty valuable to have in another place other than your personal PC, and so Git projects it and also lets other people see what you've done/do edits, which for teams could be useful. I still need to figure out how best to use it for Unity, but hey, this is a start, so I reckon the guys running the jam have a small victory here already.