I needed this tip from lexicon444...total resolution with Airpods back to normal!
This comment from lexicon444 that I haven't seen anywhere else solved my issue
I'd reset my Airpods as per every other article on the interwebs multiple times, cleaned the outside wax build up etc. etc. etc.
When I gently sucked air through it was completely blocked and following the advice above cleared out the blockages which then allowed the diaphragm to vibrate properly (assuming that gunk on the diaphragm was what was causing it to not vibrate properly).
A few minutes later I'm listening to Airpods that sound like they used to when I first got them instead of throwing them in the bin.
What is suggested above should be part of Apple's help articles.
Kudos to you lexicon444 my Daughter will massively appreciate it when she get's them on her birthday!