2013 Late Macbook Pro constantly mute completely?
Does this occur when you have stereo or headphones plugged into the jack, or only when using the computer's speakers?

If it is only when trying to use the built-in speakers, then probably there is some dirt or lint in your headphone jack outlet, and the computer thinks that something is plugged in there.

You can try compressed air (safe) or a paperclip (not as safe), or just go to the Genius Bar and let them sort it out.