I've never used it with YouTube but I have done this with JRiver. Just send the audio to the USB in audio setup and in midi setup. The HDMI will take care of itself.
MAC Mini video via HDMI & audio via USB?
Hoping someone else will have tried this. If I'm playing a YouTube video, can I send video via HDMI to a TV and at the same time send audio via USB to an external DAC? OR does the HDMI connection shut down outputs from the other ports. Thanks in advance.