How to change sound output to speakers when headphones are plugged?

Open system preferences > sound > Output check the box of show volume in the menu bar .

When head phone is plugged click on the speaker icon .

And can adjust the volume towards left or right to decrease and increase the sound by moving slider .

If we check the box of show show volume in menu bar the speaker icon will stick in the top right menu bar , but as OP mentioned , immediately if head phones are plugged the speakers disappears in that case must run PRAM .

It means that I cannot change sound output to speakers when headphones are plugged in.

That is correct.

As far as I know all Macs have always worked that way, all the way back to the original one.

Is there any 3rd party software that might get this done?

I doubt it, since the headphone port is controlled by hardware — specifically, a switch within the audio output port. Inserting anything in it (doesn't even have to be headphones) disconnects the internal speakers. If you want the ability to choose between internal speakers and an external sound output option, then use a USB headset. (edit: or BT as dialabrain suggests)

Given Apple's elimination of ports in general, I'm surprised the audio output port continues to exist.

There is a built in switch that disables audio to the speakers when you plug in headphones into the headphone jack. It works the same way as audio equipment has for decades.

You should re-read the original question.

Yes. I am absolutely right. 🙂

Understandable. 😎

No, you don't reset NVRAM. 😁

I believe the only way you can use headphones and internal speakers simultaneously is to use a USB audio device for the headphones. Otherwise you are stuck with either/or.

That's correct. Or Bluetooth headphones.