Plugin with more features and higher efficiency: https://forums.alliedmods.net/showthread.php?t=170297
The long awaited voice proximity plugin is finally released. This plugin forces voice communication only to players within a certain radius of the speaker, which makes it so you cannot communicate to players accross the map; but rather within "speaking range," which is a distance defined in a cvar. This also allows communication between teams if the players are within proximity.
This plugin has been tested in realism-based scrimmages and has proven to change the gameplay entirely. It really makes you depend on individual squad leadership, and takes the game to the next level. I don't know of any other game that has a feature like this.
Try this plugin in a scrimmage - you'll love it.
I must give credit to Twilight Suzuka as she originally came up with the idea but could not get it working.
Enables or Disables the plugin
Defines how far away you can be from a player to hear them speaking (Default 1300)