Dealing with microphone echo?
I'm not familiar with your particular set up but generally speaking, when you say "the mic echoes," do you mean that the people on the other end hear their own voices being repeated?
If so, could it perhaps be a similar issue to that experienced on some telephone calls?

"The most prevalent cause of echo in PSTN is impedance1 mismatches within the hybrid where 4-wire phone circuits are converted to 2-wire circuits. This electrically generated echo occurs when the incoming energy from the far end speaker is reflected back toward the speaker as a slightly altered and delayed replication due to impedance mismatch in the hybrid.."

More info here:
http://www.adaptivedigital.com/product/echo_cancel/echo_explain.htm