What an awesome concept, open source music.
Alas, no such thing.
/Props to Gutenberg Project and my local library.

Edit: I'm being proved wrong and loving it.

[="http://www.cockos.com/reaper/"]Reaper[/]="http://www.cockos.c…"]Reaper[/]'s download packet comes with an optional project file.

As well, under the "REAPER Mix Project" heading on this page you can find a sample project that Reaper users have submitted.

It's been years since I've gotten them, but it looks like the Acid 8Packs are still available for weekly download. The project files require you use a version of Acid (a freeware version of which is available [[url=http://="http://www.sonycrea…"]here[/]="http://www.sonycrea…"]here[/]), but you can extract the raw media files and import them into another mix application just as easily.

As is the case with many free mix-type projects, the above are not great quality in terms of signal chain and recording technique.

http://www.kompoz.com

Music collaboration site. There are a lot of projects that have tracks ready to mix. Most of the files are mp3, but the quality is generally high enough that it should work for your purpose. You will have to align the "sync tones" in your DAW.