Normally when you output audio through HDMI, you cannot control its volume from your Mac. You adjust it on the output device instead, whether it is a monitor or TV.
I’m working on my Catalina laptop while my mini is in the shop and using my newish DELL monitor without my normal speakers, which plug into the headphone jacks. With this setup, my music is loud, even when I have the monitor settings fairly low.
I was wrong.
In SoundflowerBed, I chose HDMI for the 2-channel output.
In System Preferences, I selected Soundflower (2ch) as my sound output device.
Boom. My volume control came back to life and my ears are recovering from the onslaught.
Seeing an old friend keep plugging along and doing what I need is remarkably satisfying. Thank you to everyone who has worked on Soundflower through the years, from Alexander Hudek and RogueAmoeba to MattIngalls and Maciej Wilczyński.