The best source for free sound effects and music I have found so far is FreeSFX. The sound effects and musics from FreeSFX can also be used in commercial software products. In the credits for the sound effects and musics FreeSFX must be mentioned as follows: http://www.freesfx.co.uk, when using their library.
There are also many sites, that sell royalty free music and sound effects. Personally I like very much the music collection of Shockwave-Sound.com. There is also fine collection of sound effects at Shockwave-Sound. The customer service of Shockwave-Sound is also excellent. Remember to choose the right kind of a license when buying.
Also great site for music to YouTube videos!
On my videos page in all the of the videos the music is from Shockwave-sound except in the Ball Shooter videos.