Are you getting audio if you use Volumio to stream from a file share/local file?
I don’t think by default you can just use it as a USB audio device, it’s not something I’ve ever tried.
You have a couple of options for sending audio from the PC wirelessly, either DLNA or Apple airplay.
Both have to be enabled in volumio, and I think DLNA support is a plugin.
They both require software support or something like “Stream what you hear”.
In case it’s useful here are my playback options
