Does airport express support USB audio devices?

no, usb amps and speakers are really external sound cards and that mean they require a device driver installed to operate something one can do on a computer, but not really on a router or even an iOS device

The USB port on the AirPort Express will only support a compatible USB printer. Other types of devices are not supported.

No. The AirPort Express base station's AirPlay functionality will only work to receive an AirPlay stream from an AirPlay server and sent it out the base station's audio port.

Apple has always been pretty clear about the USB port on the AirPort Express. Check the Specifications on the AirPort Express to confirm.

Mac - AirPort Express - Technical Specifications - Apple

The iOS device would be performing as an AirPlay "server," whereas, the Express would be performing as an AirPlay "speaker." To related, by very different functions required for AirPlay to work properly. AirPlay is an IP protocol suite used for streaming audio over a wireless or wired network.