I had this problem during setup of a new iphone 12 pro max where the ringtone volume was v low despite the volume being turned all the way up on Sound and Haptics and despite Vibrate on Ring being activated- the phone did not vibrate and the ring tone volume was slight and descending. Even after an hour of sound testing and tech support with AppleCare, (tests on left and right speaker volume levels) and the Apple tech believe this ringtone volume issue to be a hardware issue and intimated that a new replacement iphone might be needed. I decided to quickly google and Brooklynrican's answer saved the day - was spot on correct:
"Go to settings, then Accessibility.
select Face ID & Attention.
in the middle you’ll see “Attention Aware Features”. Turn this off.
THANK YOU Brooklynrican. I think the problem may have originated during the SET UP of FACE ID step which we did not establish properly and so ended up skipping - perhaps this somehow turned on the Attention Aware feature which resulted in descending and low ringtone and no vibration. Hopefully Apple will put in a fix for this with the next update as it was perplexing even for their tech folks to figure out.