Hi all,
A little update on things. For reference, my 2017 MacBook 12" (i7, 16gb ram) is running MacOS 12.2.1.
I initially commented that I had luck changing the bitrates of streaming. That "fix" lasted for mere hours, if that, and not the Sound Check nor anything else had any effect.
I had been running sound from my external monitor to speakers via AUX out of a port on the monitor. The sound was delivered from the MacBook to the monitor through the HDMI, which comes through a USB-C dongle. I noticed that the volume differences were present during Apple Music, Apple TV, and Netflix (via Safari), so it wasn't an Apple Music setting. I also found it unusual that my MacBook handed over complete control of the audio signal to the monitor, which I am sure is a feature of that functionality, but it made me wonder if that lack of volume control on the MacBook's side contributed to the inconsistency of the audio being delivered over HDMI.
Considering the fact that I've had an outsize number of problems with HDCP compliance of HDCP-enabled external monitors in past versions of MacOS with this MacBook, I believe that the audio delivery via HDMI was the problem, on the MacBook's part. I currently have had my speakers directly connected to my MacBook's headphone jack port for the past two-ish days. I've had no problem since and every finger and toe my soul has is crossed that the current state of things doesn't change.
So, the fix is simply:
If your audio is currently running through a monitor's HDMI connection, use aux through the headphone jack instead. I do not know if audio through a Thunderbolt or USB-C hub has the same problem.