Does the USB port transmit any audio signal?

No, it doesn't. The USB interface is a remote connection for the Torpedo Remote Software: you can control all the parameters of the Torpedo, upload and manage new cabinets, upload new firmware when needed.

You can use either the Line out or the S/PDIF out to go into your mixer/DAW (assuming you have a Line and/or a S/PDIF input on those products).