Try using the Windows Sound Recorder to record a short audio clip to
see if you still get the echoes. This will at least help isolate
whether the issue is with PowerPoint or your microphone/sound card
setup.
On Windows XP Sound Recorder is located at:
Start->All Programs->Accessories->Entertainment->Sound Recorder OR
you can do a Run->sndrec32.exe
You asked the same question on March 21 and I gave an answer on March 22. Of
which I do not know, whether it was helpful or not. So why do you start a
new thread? Please stay in the same thread and reply what happened when you
tried that suggestion.