Hello, and welcome to the forums.
This could be any number of things. People playing this game often use other, third party voice-IP applications to talk with their friends while gaming, or with the rest of the team on certain servers.
They do this using programs like TeamSpeak and Ventrilo, which are mainly for clans and communities, and usually not intended to be used as a radio for the entire team.
There is also another voice program that is widely used in the PR community, which is called Mumble. The PR version is called PRMumble 0.5.
With this program, you can connect to the Mumble channel of a server, and talk to everyone on the team or server in that channel, if they are near to you in-game. This is because Mumble has a directional audio feature, which is supposed to simulate speech.
There is also a radio functionality for the squad leaders.
I recommend you check out this guide to set up and learn more about Mumble:
https://www.realitymod.com/forum/f10-pr ... ality.html
There is also a good, and somewhat more detailed guide on the Alpha Project Forums, here:
Alpha Project Mumble Guide - Alpha Project
If you have any questions, I'm sure people will be more than happy to answer them in this thread!
EDIT: There is also the in-game Squad Leader to Commander and vice versa radio channel, which is built into the game.