If you upload a video to Zooppa and it has music (note, you can make my job easier and choose not to include music :-p), that music must either be:
1. An original piece made by you.
2. An original piece that you have the expressed written consent to use.
3. Licensed royalty-free music.
NOTE: just because it has the word free in there doesn’t mean it is. There is a fee to use such music in a commercial venture like a contest.
One of the most popular sites is incompetech.com each song on there is $30.00 to use. This is the one I would recommend too. It’s a flat rate and the music is decent.
One thing I brought up on the forums (which no one has responded to as of yet) is the idea of going to fiverr.com‘s music and audio section. Fiverr.com is a place where people offer some sort of service for 5 bucks. If someone is adventurous enough to use fiverr for their project let me know. I’m really interested to know how that works out. I might use them for a video brief. Even if you do go with fiverr.com I would also get a synchronization license or a statement from the artist saying they allow you to use their work for the purposes of the Zooppa contest.