Second Life runs the OpenAL audio engine under the hood. But barely utilises any of its features.

One such feature is hardware reverberation zones, essentially areas that once entered will alter all the sounds within to have echo, bounce, and other features. These are all outlined in the OpenAL specifications for 'Level 2' signal processing. Second Life already utilises this for the Doppler effect.

This allows custom definitions of sound for creating realistic soundscapes like caves, halls, theatres and more. This would enrich the immersiveness of Second Life by a lot.

In Second Life, this could be implemented similarly to reflection probes. Prims, either spheres or Cubes, set to 'Reverb Zone' areas with configurable parameters for echo, dissipation, absorption and so on.

Someone else has already suggested 'audio zones' where the sounds are muffled inside certain prims. I'd like to add that these settings could be bundled in the same prim as some sort of 'Advanced Audio area' setting, perhaps in the 'Features' tab.