I am also interested in this feature.
It could be a case that replaces the normal one but without any holes for speaker/usb/audio jack. So the only way to use it would be through bluetooth or the speaker through soft plastic. (I guess the sound would be lower but still usable)
I’m imagining a case with a hard backside but a soft(er) front that still allows for the insertion of the phone and some sort of sip-lock/roll-lock.
When you need a phone to be waterproof you don’t need the charger/headphones or any other peripherals. The beauty of this compared to other universal waterproof bags would be the size.