USB Soundcard Device Class Specification document

I'm looking for a specific document.

I was reading the 'USB Audio Device Class Specification for Basic Audio Devices' document (http://www.usb.org/developers/docs/devclass_docs/audio10.pdf). In section 1.1 in that document, it is mentioned that "More complex audio devices, such as USB soundcard devices are not part of this specification".

Can someone point me to a document that does have the specification for USB soundcard devices? Do I need to be a member of the USB-IF to get this information?