In my early times I’ve used FreeSFX for example in Ball Shooter. 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.
One of the probably finest sites for free music and sound effects is Soundimage.org. There are simply amazing compositions for free, you only need to put attribution information to your product.
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!