FAF has its own mumble server that you can use to talk to your friends.
Connect to the FAF Mumble server:
- download+install Mumble, for example from http://www.mumble.com/mumble-download.php
- start mumble and connect to the FAF server at faforever.com, port is 64738
FAF offers automatic channel switching, to automatically group up with players from your team who also have this setting enabled. To enable it, check Options in the toolbar at the top of the FAF lobby, go to Voice and enable both options there