This is correct, it's called 'Audio Device Class', specs here [1][2]. It's said that the new enhancements that Intel is pondering add power-management features to this spec, but I can't find much documentation on it, other than rumours.

[1] http://www.usb.org/developers/docs/devclass_docs/audio10.pdf

[2] http://www.usb.org/developers/docs/devclass_docs/frmts10.pdf