For my own project (much simpler—only deals with stereo audio), I used this: https://www.dropbox.com/s/t5dvah8zke3uky8/HellOfAGuy_demo.mp...

Audio came from here, with a very open license: http://www.audiogroup.web.fh-koeln.de/anechoic.html.

It seems like there's a possibility you could work with one of these artists in some kind of promotion deal. You could have limited distribution of their track as a test app, and they could get a little promotion along with your cool space-age idea. I bet a nice email to their PR people would be worth it.

I am not from Germany, but I seem to remember reading that the GEMA licensing is mandatory even for bands that give their music away for free. Can anyone clarify if that's the case?

You can use my tracks for free if you want: https://soundcloud.com/lifeformed

Use some creative commons songs.

Great idea, thanks :) I used my favorite CC song, which is Pocketpod - Hybrid:

Original: http://www.hajoheadphone.com/assets/417586_hybrid_orig.mp3 Virtualization: http://www.hajoheadphone.com/assets/417586_hybrid_hahe.mp3

There's lots of distortion (like digital clipping but low-passed, if that makes sense) at the louder parts of the song. It also seems that the processed sample has its dynamic range reduced by a lot.

Thanks for letting the non OS-X'ers in! :)