Crap. I'm the one with misinformation!
Sorry, I was thinking of the 3rd generation Mac Pro.
10.7 is in fact the newest supported OS. I believe the issue is the EFI firmware locked to 32 bit.
The Mac Pro didn't turn into the monster until the i7 came along.
However, it's still a supercomputer and will do all the things I said. Maybe its CPUs will get above idle with only a 100 track studio mix project. Still overkill for a typical DAW setup.
There is a solution to install 10.8 or 10.9 however, if you are interested in a more Windows-like level of DIY for an install. See here:
http://forums.macrumors.com/showpost...1&postcount=26
Is the issue that you have some plugin that requires 10.8 or later to install? What is it?