Using external microphone disables internal speaker 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?
Since I posted this question, I got a new external monitor that connects to the DisplayPort (Acer EB321HQU). It has its own speaker, so the Sound menu allows me to choose between Internal Speakers and the monitor.
When I plug in the microphone, the Internal Speakers choice changes to Headphone, so it still disables the built-in speakers. But the option to use the monitor as the output device is still available. So I can select that and get audio without a headphone. Plugging in the microphone automatically changes the selection from the monitor to Headphone, but I can easily switch it.
This solution won't work more generally, it's only for configurations with a monitor that can be used as a sound output device.