Sound Check not working for you?
It didn't for me either but I think I may have found a solution. I get heartily sick of adjusting the volume to correct for too loud or too quiet tracks, something I have had to do frequently as I usually shuffle play my entire music collection. Sound Check just didn't seem to work.
However, I got a clue from Spotify of all places. On Spotify, if you are a paid subscriber, they too have a volume normalisation service, which at first glance, doesn't appear to work either. However, Spotify has one further setting - you can set the overall playback level to 'Loud', 'Normal' or 'Quiet'. They recommend 'Quiet' if you want to preserve the finer nuances of the music - so I tried it! Hey Presto! Volume normalised as well! It would appear that their normalisation process, quietens loud tracks but does little or nothing for quiet ones so it improves the experience a bit but quiet tracks remain quiet. Setting the overall playback level to 'Quiet' would appear to level the playing field.
This got me thinking. Does Apple Sound Check work the same way? So I'm trying an experiment. On my MacBook, I highlighted all 15,000 tracks and then right-clicked. I selected 'Get Info', then 'Options' and then reduced the volume by 40%. Then Clicked OK. It took about 10 minutes to adjust all 15,000 tracks.
I then forced a Cloud Library update (File, Library, Update iCloud Library). This took a few more minutes but Bish Bash Bosh... Volume levels now appear to be normalised, not just on my MacBook but also my iPhone AND my Homepods.
Early days but so far so good. Please post back here if you have similar success - or problems!