Change volume of speakers before unplugging headphones
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: