You can use assets, but if it's from an old official game and you don't have the permission to use the music, it wouldn't be ok.
If a team wins the prize, the person submitted would get contacted, of course.
Might be a good idea to discuss with the teammates internal who would want to take it, if that happens.
The other team members can have a free sticker pack send to each of them, if wanted. (Same pack as place 5-10 will recieve.)