3.5mm mic: how to explain my MacBook that it's just a mic and not an headphone set
You do not have a microphone jack. You plugged your microphone in the headphone jack. This is why you can not select internal speakers.

Try one of these adaptors:

USB Type A:

https://www.amazon.com/customerpicks/Explore-usb-adapters-for-microphone/49a722acca2e7546619b

USB Type C:

https://www.amazon.com/CableCreation-External-Headphone-Microphone-Compatible/dp/B075YFDJ4Q

https://www.amazon.com/Headphone-Microphone-Plantronics-Sennheiser-Compatible/dp/B07KPLDJ18

Or get a USB microphone and skip the adaptor.

https://www.amazon.com/usb-microphone/s?k=usb+microphone

https://www.amazon.com/usb-c-microphone/s?k=usb-c+microphone

Perhaps the jack is for a headset (combination microphone/headphone).

There is this question: How to use a wired headphone AND a mic at the same time on the Macbook Pro Retina?. The answer would suggest the jack would accept a plug with two rings. This would allow the jack to be used for a mono microphone and stereo headphones. If this is your arrangement, then the you still would need a USB solution since using the jack would not allow the internal speakers to be used.



Although, you have to wonder. If you used the microphone jack and left the headphone jack unused of an adaptor, like the one shown below, then could the internal speakers be accessed? My guess is probably not.