Bah! I should have known what talkback was. I have a set of Tritton AX Pros without the digital box and my friend has a set with it. It's been a long time since I've gamed with them on, and I've tried a few pairs since, but if I remember correctly I could hear myself yakking away in them.
If I get a chance I'll put them on tomorrow and let you know, and I'll ask my friend if his have talkback as well..... in case there's a difference.