At various conferences around the world I have heard from big podcasters and youtube themselves – they stress to beware with using copyrighted sound in your youtube video.
If you use a commercial music soundtrack to your video, they then own the rights to it. Youtube constantly scans the audio of videos to find copyrighted music, you may get your video deleted, audio disabled and if you do not own the copyright of the sound, that means you cannot earn from the video and youtube instead would allow the music copyright holder to serve ads against the video.
Here is a list of resources for to find Royalty Free Music for Youtube Videos and Creative Commons Audio for Podcasts:
Jamendo – Has almost 300,000 tracks of all genres
Beat Suite – Search Royalty Free Music on Beat Suite