Please see this link: avoiding feedback with VoiceSolo.
If you are unable to watch the video by clickin the link above, and wish to download it:
Windows: right click on the link and select "save link as".
Mac: control-click on the link (or right click if you're using a 2 button mouse) and select "save link as".
If you don't have a video player capable of playing back the file, give the free VLC media player a try.