I have a Mid-2015 Macbook Pro, which has the combined headphone/microphone jack.
I normally use it in clamshell mode, and in the past some people have had a hard time hearing me if I try to use the built-in microphone, so I got an external microphone. I was just testing it, and discovered that when I plug it in, the Mac thinks I also have headphones plugged in, and disables the internal speaker.
When I go into Sound Preferences, "Internal Speakers" is replaced with "Headphones" whenever I plug in the microphone. I guess it thinks my microphone is actually a headset with both microphone and headphones.
Is there any way to disabuse it of this notion, and enable the internal speaker while using the external mic?