What about a late 2010 Lincoln Town Car, is that quiet enough?
People with hyperacusis are also reactive to vibration and shock so a Prius would be bad. Electric doesn't solve the problem of road noise or motorcycles passing. For that you need double-pane glass and really good insulation.
I was also thinking of a Lexus LS430.
You know you can have a form filled out by your PCP from the NTSB so you legally can have your airbags disabled. Reason being with earmuffs on the impact of the airbag would cause blunt force trauma to head and possibly death as a result.
This thread seems to be Europe based and I'm in the US so those decibel charts don't cover cars here.