I did some digging and Audiosurf uses .cgr models.
After a quick google search I found out these are Quest3D Models.
Quest 3D even has a pic of audiosurf on their site
Yeah, most threads in the Feature Requests section mention Quest3D as a means to an end for most requests. Audiosurf is made with Quest3D.
The .cgr files are protected though, so you can't actually open them in Quest3D. There was a thread where some guy claimed to have changed the .cgr itself and modded the game a bit, but he never said how, or hinted at how to remove the Quest3D protection.
I haven't been bothered to open a hex editor, and probably won't be bothered to do so.
Audiosurf