Try this suggestion by Jim Cookman:
"Jim Cookman Milford, CT
Re: Can I damp down echoes a bit?
Jan 13, 2010 2:50 PM
There is actually one desperation method you can try, but it'll take a little experimentation.
Set up a mike right in front of one of your monitor speakers. Hang a damp dish towel over the speaker and re-record your audio. If the towel is too thick or too wet, you'll get crap, but if you play around you can actually lose quite a bit of echo as the weaker signal will be damped down-- to coin a phrase.
You'll want to get the monitor and mike as far away from the computer as you can get-- or even dump the audio to a cd and record from your stereo system. The fan noise will wreck the whole thing.
Last ditch, but it can work."