Apple - How do I control Macbook volume when a DP display is connected?
- MonitorControl Control your external monitor brightness & volume on your Mac. It does not require installing a driver.
- SoundFlower MacOS system extension that allows applications to pass audio to other applications.
It's because Apple computers don't support CEC (Consumer Electronics Control).
Basically, macOS doesn't allow you to control end devices attached via HDMI or DisplayPort. Changing the volume would be considered "controlling the device"
I did the 14 day trial of SoundControl which did work, but I wasn't really willing to pay $15 for something that I feel should be available out of the box for all MacOS machines.
After some more digging I came across eqMac2. Quick install, free and open source, and worked straight away. Highly recommend anyone reading this to give it a try.