The bad part is that at this rate I'll never use the stuff I'm finding, but the good part is that I haven't locked into a particular way of getting these audio files integrated.
In my search I came across the Skyland Games blog and their suggestion to use Spotify to add music to your game. There are several music streaming services to choose from and I'm sure there are advantages & disadvantages to just about every combination of free or paid music streaming services.
What helps put Spotify over the top in this instance is the fact that Skyland Games has a specific post listing more links to Spotify songs than I care to count. They've broken up the links into six main categories:
If you have an extra computer, tablet, or smartphone and the appropriate speaker you should be able to use Spotify to "music up" your game. If you are looking to actually purchase music for your use, then at the very least this listing could be used to get a head-start on useful songs/artists to acquire.