I have a 2015 MacBook Pro on the latest version of High Sierra. I often use a USB microphone for Skype, etc. When I plug in the microphone, my Mac automatically detects it as an input source. This is good. However, it also automatically detects it as an output source, which is not good at all, since the microphone has no output capabilities. Every time I plug in the microphone, I have to go to Sound preferences and unselect it as the output.