Q: How can I talk to another user on my own wifi network?

I'm wracking my brains here, but I just can't work this one out. Is there any way under OS X (I'm on 10.9) to plug in a headset with microphone so that I can talk (using my physical vocal cords!) to another user on my own household wifi network? The scenario is that my son is asking me to play Minecraft with him all the time. We have Realms, so we can play together no problem - we just can't communicate properly whilst doing it as the two Macs are in opposite ends of the house. I have a bluetooth headset and can easily get another for him - but what then? How do I get my voice to travel over the network to him, and vice versa? Apparently iChat used to do it, but that's gone. I can't use FaceTime, as my MacPro doesn't have video. He's on an iMac that does, but I can't even get my iPhone to connect over FaceTime to it, so that's a non-starter. Surely it shouldn't be so hard? Help!