Settings to turn a prim or mesh(though could be limited to simple prims similar to reflection probes if anything else would be too complex) to make the object into a "Sound Dampening Volume", which works similarly to how sound works under the water volume in SL, muffling sound sources that are outside the volume, while the avatar's view is inside the volume, and vice-versa. Maybe including some simple settings that allow you to control the level of muffling as appropriate for the use of it. IE less muffling for a shack, medium for inside a car, or heavy muffling for inside a thick solid building or cave etc.

Example use: A car radio with a volume set up around the main cabin of the vehicle, and the radio inside the volume, so if you drive while in mouselook, the music is clear for you, but muffled to anyone with their view outside the vehicle.