There are a few different options for where you can get sound and music. You can make the audio yourself, you can hire composers, you can purchase stock audio, or you can even use free stock audio.
If you purchase stock audio or use free stock audio, be sure to check the licensing agreement. Purchasing audio does not automatically guarantee you unrestricted use and sometimes, even if you pay for it, you cannot use it in a commercial game.
If you're like me, free is probably the price range you are looking for. What follows is a list of a few places where you can get free resources to help you design your own audio or get pre-existing sound effects/music:
Audacity is free software and can be downloaded at http://www.audacityteam.org/. This software can be great for recording your own sound effects and dialog.
My favorite place to get music is incompetech.com/. You can sort the music by feel and genre and there is a huge selection of quality...