Why Doesn't Anyone Hear Me In A Yahoo Conference?

I am inquiring for a friend....people can hear his voice in one on one im's and pc to pc calls but they cannot hear him in a conference with hands free. His name and voice recognition bars show up but no one can hear him when on hands free. He has tried everything including uninstalling and reinstalling. He has performed voice test troubleshooter. The only way he can be heard in a conference is to place a pc to pc call and be heard through someones elses mic. Do you have any suggestions?