BenSound - BenSound offers high-quality, production-ready, free music tracks. You can also find free music from other sources, though the quality and usefulness of their tracks are much less consistent. These other sources include the Free Music Archive, the Vimeo Music Store and Incompetech.

Audio Jungle - Download the perfect musical track for your app or video. An average track costs $19 to license.

Universal Sound FX - Sound design is an integral part of any quality app. Universal Sound FX is a library of 4,000 high quality sound effects that you can purchase for only $40. The tracks are specifically designed for use in games, however you must install a free copy of Unity in order to download them. If you want a quick solution for just a couple sound effects, SoundSnap offers sound effects priced from $1-3, and FreeSFX offers great free sound effects.