Ok. Give this a shot. I actually resolved this yesterday by doing this:
1. Factory reset - when setting up again I chose US as a region.
2. Enable developer options - disable absolute volume
3. Enable engineering mode (dial pad and dial *#*#3646633#*#* scroll right to 'hardware testing" - audio...
I can spare you some time. The audio on Xiaomi 11t is simply horrible. This is my first Xiaomi experience and I'm at the point I want to return it. I spent maybe 6 hours total on this and nothing works. Greyed out options won't do nothing for you in terms of loudness and will only open up with...