[kevinh]

I'm not sure if anything has it built in, but Thief: The Dark Project (in 1998) had sound propagation. There's some discussion about it here: https://www.ttlg.com/forums/showthread.php?t=151206. I wonder if you could query Unity's navmesh to calculate the distance, but that wouldn't work with, say, someone on the second floor hearing someone outside through the window.

[lytedev]

Wouldn't you have an alternate navmesh for sounds with the appropriate "cost" heuristics based on how well sounds propagates through the mesh's material and across media?

[jjmarr]

This sounds like a great use of hardware accelerated raytracing. The problem itself is very similar. Sound attenuation can be modelled as transparency.