Reliable method of detecting voice on channel

[qwertos]

Looking for a reliable way to detect if there is a user talking in the current channel.

Use case: I originally began looking at this project for using mumble in RoIP (R = Radio) situations. Because of the nature of radio, it is necessary to detect when to start and stop transmitting.

I figure the best way to implement this feature is to create a call back system that notifies a function when a user has begun talking and again when finished. I have looked into doing this and pushing it upstream, but I can't figure out a way to have this information be reliable.

Thoughts? Suggestions?