So I finally arrived at a wonderful simple and robust solution. Run usb C to HDMI adapter directly from laptop usb C port, and run HDMI to Yamaha multichannel receiver. The Yamaha decodes 5.1 signal to my five speakers, and I can turn on a projector from the receiver to actually add a fourth monitor when needed. System Preferences > Sound > Output detects the Yamaha, or the projector when switched on, and runs the audio through speakers regardless. The detection by laptop is automatic, it works like a charm.
This is an obvious and simple solution, and have seen it suggested also on the link Brody sent. Unfortunately it did not work when I first tried it last year, hence I was flummoxed. I will share my troubleshooting in case it helps anyone else hitting obstacles. It is essential to be able to monitor 5.1 if we are mixing multichannel audio!
Since HDMI carries multichannel audio when available to compatible devices, I had originally put an HDMI switcher to one of my monitors, so I could also send the signal to a Yamaha multichannel receiver if desired... decoding the 5.1 audio, and sending picture to projector when desired.
Unfortunately it did not work when I first tried it... the display could not be detected, and I do not recall the audio working either.... even if it did, it could only provide audio monitoring while switching away from computer monitor. I was baffled, but thought the Yamaha receiver was not something a mac could detect and adapt to. I did run a cable from switcher to projector, with an optical-out splitter in between to run TOSlink audio to the Yamaha receiver. Somewhere along the way the optical-out splitter ceased working, so I really preferred to just send HDMI to Yamaha receiver... except it could not be detected.
Then I realised just yesterday that the hub might be the issue.
All of this was via a hub (due to only four usb C ports on MBP15) and the HDMI switcher, and the hub works very well in carrying HDMI, ethernet, usb 3, SD cards (hence i originally did not think it would be an issue). I tested without switcher and problem persisted.
So I had the brainstorm to try a usb C to HDMI adapter direct from one of the usb C ports (I resisted this as I like to keep at least two usb C ports free for portable drives), instead of a hub. Et voila, all works like a charm as mentioned above. I have one less usb C port available, and am now exploring if I can now safely and reliable run more than one portable drive from a single USB C port.
BTW, having eliminated need for using AUX input on the Yamaha receiver, there is no hum... very clean sound.
Hope this helps someone, and thanks again Brody for the input! 😊
