Guess what? I turned the monitor and the Mac off and let it rest for a few minutes, and turned it back on.  The volume setting worked this time.  Maybe the firmware needed the computer to reboot...
I have good news and bad news for [NAME].  First of all, I feel your pain. I've spent hours setting up this monitor. I bought a Mac Mini last week and then bought a DELL S2422HZ monitor from Best Buy since the Dell website said it was Mac compatible. The Apple staff person at the Apple Store had told me most any monitor would work, and I liked the promise of a built in webcam and speakers on this monitor. The Apple guy said even a TV with HDMI port would work as a display device. Mac setup said the best display connection for this display would be HDMI to HDMI. DELL setup instructions said either HDMI or the USB to USB would work. I first made the mistake of connecting both cables at the same time. After some discussion with Mac users online, I removed the HDMI and just left the USB-C to USB-C connection. Good news: Mac Mini desktop showed up on the DELL. Monitor.   Then I updated the monitor's firmware using my Mac, as mentioned earlier in this thread. I also updated the Mac's OS.  And installed an antivirus program on the Mac. I set up a Zoom meeting to see if the webcam and speakers would work. Webcam works GREAT!  But Sound was VERY low. The sound on the Mac is set at 100%, but if I wanted sound to come from the monitor, I knew I had to find a monitor setting somewhere. Bad news for me is that, though I found the volume adjustment in the menu on the monitor (you have to use those cute little buttons under the lower right of the screen's bezel), I saw it was set at 30%, but can't figure out yet how to increase that.   If a DELL staff person reads this, would you tell me how to increase the %, please?  This might help Angela828, too.
S2422HZ, Mac Mini, Zoom speaker volume is low