Default System Sound Location
All sounds are .aiff and you'll find many by going to:

Mcintosh HD>system>library>components>

Then right click on CoreAudio.component and click 'show package contents'

Then go into Contents>Resources>SystemSounds>finder>

Note that this is different in El Capitan but is what I used in several OS's before, ie: Yosemite, Maverick, etc