By default, the Mac will send the sound to channels 1&2. Make sure that you’re looking there for the sound. Also, make sure that you set those channels to USB input.
One last thing to check. Make sure your volume is up. When you change outputs, sometimes the Mac sets the volume to 0.
If you need to run the output to a different channel, this is accomplished through the Audio Midi Setup program. Just search for it and make sure you select the output side, not the input side.
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