Hi Al,
Yes, only protocol masters can be USB devices. This is mainly because of the wireless clock distribution. The USB host (e.g. the PC) is normally clock master, and in the PPW protocol only the protocol master can be network clock master: a PPW USB master gets the clock from the USB host, and delivers this clock to the protocol slaves in the network. Hypothetically, if a slave should be plugged into the USB port this USB host would try to give its clock to the protocol slave, but this slave should get the clock from the PPW master, not from somewhere else.
That being said, it is totaly possible to feed audio to the USB host. The PPW USB master can be configured to be e.g. a headset base station where audio is going both ways on the USB interface (both into and out of the PC). The PPW USB master can also be a microphone base station receiving audio streams from the slaves and feed this audio to the PC over USB.
-Kristoffer