I have been asked where do you sounds for apps. Well here are a few sites I have used for sound/music sources recently:
http://soundbible.com/royalty-free-sounds-1.html
http://audiojungle.net/
(I am sure I have forgotten some. I will edit when I remember!)
Those are the sites I start with. There is a LOT to be had right there and I usually don't have to look far. After that Audacity/Sound Forge apps comes in handy making new sound combos derived from other sounds. Its fun to play with, but I spend WAY to much time on it when I get into it!
I want to mention this last one, it has a reasonable cost to consider, but I find their samples to be great to try out entire packages of sounds. The guy running it has been very receptive to my feedback. Overall the polish is there. The need to rework sounds is non-existent. I have yet to buy set of sounds from him (no new projects in need yet), but it would save me countless hours of surfing and amateur foley work (did you know snapping a bundle of celery makes a great bone breaking sound?) Audio Skins is on my short list for sure!
http://www.audioskins.com/index.php <- Definitely worth checking out!
Side note: Remember to read and understand distribution your license allows (royalty free or not!) That is a topic for another post another day!
Do you have a favorite sound site? Commercial or not, please share in the comments! I'd love to hear it!