I don’t know if it’ll work with RDP, but the secure way to pass your USB mic to Whonix is to create an audio qube with your USB mic attached to it and attach the virtual dom0:mic device to the Whonix:
I don’t know if it’ll work with RDP, but the secure way to pass your USB mic to Whonix is to create an audio qube with your USB mic attached to it and attach the virtual dom0:mic device to the Whonix: