That resolved the problem - audio playback then worked as expected. I've been waiting on software updates or whatever, but nothing's happening. Audio/Midi setup & sound preferences says that it's a 'Digital Out' not a 'Headphone out'. No sound through headphone, itunes stops, pointer turns into spinning circle. So the solution turned out to be: copy wdma_usb.inf from another Windows 7 machine back to the \windows\inf directory, then go to device manager, “upgrade driver” on the audio device, and explicitly point it back at the \windows\inf directory to find the inf. Brand new out of the box, this thing has a faulty headphone out. The problem turned out to be that somehow on this system, Windows 7’s built-in INF for USB audio, which should have been located at \windows\inf\wdma_usb.inf, was missing (either because the machine was upgraded from Vista to Win7, or because of some other audio work an IT helper had done months earlier).
In Device Manager for the USB audio device, the status was “The drivers for this device are not installed (Code 28).” This is unusual, since the CM6300 chip in the dock uses the built-in Windows audio class drivers - there are no other drivers to install. The apparent problem was Windows would not match any driver to the audio function on the dock. For the benefit of others, want to post what the problem and solution ended up being.
