Good news, buddies, gmod dev person AzuiSleet has added
locational voicechat stuff to gmod.
No information yet on exactly how it works, if servers can force it on, configure the range, and other such things.
It's going to be controlled in the LUA PlayerCanHearPlayersVoice hook. Right now it returns a boolean indicating whether Listener should hear Talker, but if this modification is added to Gmod, it will probably also return a boolean indicating whether you want the listener to hear it as positional.
It's that simple because the Source engine already supports positional voice and it's enabled almost exactly like this in the C++ source code. :P However, range can't really be controlled. From what I can tell, positional voice works just like normal Source sounds do, but there doesn't seem to be any way of controlling voice volume or how far it travels. :(
http://twitter.com/FacepunchBot/status/17229673877