Audacity is a free open source digital audio editor and recording computer software application, available for Windows, Mac OS X, Linux and other operating systems. Audacity can be used for post-processing of all types of audio, including podcasts by adding effects such as normalization, trimming, and fading in and out.
Free-loops much like Freesound is a collaborative database of audio snippets, samples and loops openly licensed for use. There is a wide array of audio available but most of the categories are built around instrumentation, so this could be a great resources for electronic musicians and audio programmers.
Freesound has less audio in full song format and more in the way of audio snippets, samples, recordings, bleeps and loops openly licensed for use. This is actually where I ended up finding the audio I needed for my project. There is a wide variety of audio available here from samples of nature and industry, to audio jingles and riffs.
The Free Music Archive is described as an archive of public domain and Creative Commons licensed songs managed with a curatorial approach. The bulk of the audio available here is in the form of full songs from artists around the world. The Free Music Archive gives up and coming artists a place to share their bio and link to their home page so that users can learn more about the music they discover.