I have a 15" Macbook Pro mid-2015.
If my headphones are currently plugged into the audio jack, is it possible for me to control the volume of the speakers?
So that, after unplugging my headphones, the speakers can already be muted for example?
If you had already set your speaker to be mute before you plug in you device into your audio-jack port after you unplug your headphones it will still be mute.
Here's an example.
Before the headphone jack:
You can see that the speakers are set on mute.
When you plug the headphone it will be on the last set volume:
And again when you unplug them, you can see that they are still on mute: