If you use music or sound samples in your animation, you must own the rights to use the music/sound. The same also applies to images, or any other material you incorporate into your animation.
If you don't have rights to use the material, then, if you win a prize in the Animation09 competition, and your work is displayed publically, you may find yourself in breach of copyright. This will cause big problems for you, and for us (The University of Manchester) as "publishers" of your material.
For ideas about sources of freely-usable music and sounds, this article from The Guardian is a good start, and has lots of pointers: http://lifeandhealth.guardian.co.uk/guides/makingvideo/story/0,,2245025,00.html.
We'd welcome hearing any ideas about other sources. Please post here, and share with others.
Cheers
Toby