Just like the sound physics mod, the VOIP client would have to know the hardness of the surrounding materials, which could be configured. An input config string or config file could help streamline things for players. As for the map data itself, some games have extractable map data, for others a user provided 3-d replica could be used. Having multiple input methods for map data would increase compatibility between games. In the rare instance where map data is both variable and non-extractable it may not be possible.
For data transmission reverb can be applied before transmitting, and occlusion and attenuation can be processed on the other end given the proper coordinates. I don't think any additional data would need to be sent. Occlusion is a cheap algorithm to run, depending on how many lines of reflection you want. More reflection means more realistic sound bouncing around corners, but also higher processing requirements. Mumble already does attenuation.