Setting the mood through music opens up what experiences are capable for your user. But choosing the right sound file for your game can be tough. Most sound files are copyright protected and unavailable for use if you want to make a profit on your idea. However, there are a few sites out there which I’ve found to be useful:
freesound - a great resource for sound effects. It has everything from thunder to restaurant noises.
ccMixter – lots of music files for various genres.
incompetech – great background music. Some are created especially for game use.