There was an article on Major Nelson about it. Normally if you hear it its not you. The person that isnt getting it needs to clean off the top of their mic plug.
Another thing that can cause it is someone with their sound up way too loud, with voices set to TV instead of headset.
Nah, seemed about the same, I think with DoA4 its hard to not only tell who is the bottleneck, but also if its really just the game about to crash haha.
I can confirm what Kryptonian is saying is true. I had the same problem with my mic. It was causing others to hear an echo whenever I joined a game. I first heard about this from a friend at Microsoft who told me that some headsets were manufactured which had a coating on the tip of the headset (where you plug it into the controller). This thin coating could be wiped off and the problem would go away. I did that, and I hear no more complaints from Xbox live players about an echo when I join.