I've finished the research I had been working on: scrubbing the web for some quality sites to find free music and sound effects for XNA game development.  

Similar to what I encountered when looking for free sprites and tilesets, many of the sites out there are of dubious legality.  Luckily, however, there are places with legitimate free content that can be used in commercial games and I've updated my Free Game Assets article with the quality free audio I found.  Each listing in the article indicates whether the site has music and/or sound effects, includes a brief summary of the site, and contains links to the free content as well as a direct link to the site's licensing page when possible.

Hope you find the information useful!