This problem is nearly five years old and I've yet to find a single solution. I'm using a 2015 macbook air but the problem occurs on older macbooks as well. My system sounds (eg. youtube videos, other people's voices if I'm in a skype/etc. call) are being played through my built-in microphone. Other people can hear videos I watch, and their own voices, which are also being played much louder than my own voice. As a result, I cannot use the built in mic to voice chat because it makes it unbearable for the other person. I can't find anything to fix this in sound settings or in the Audio MIDI Setup. In that app as well, I see that there are "2 inputs" for the built in microphone, one of which I assume is my system sounds? But there is no way to test this or change any settings, most of them are grayed out and cannot be changed. I didn't change any settings, this computer is nearly brand new and came with the problem. Please, does anyone have a solution or any advice.