It's okay, my typewriter doesn't have a mute button either.
Back in my day, we just took a pen and notepad to class for taking down notes or doodling. These days, it's about notebooks (and that'd be of the laptop variety). Nearly everyone can type faster than they can write by hand, so naturally using a computer is a better class-aid.
But what happens if you don't have access to a laptop for a day, but still prefer the old QWERTY experience? You bring a typewriter, of course.
That's exactly what some young student did, much to the amusement of his peers. We can't exactly make out what the professor giving the lecture says to the student, but we're fairly certain that he asks him to "mute" the sound of his typewriter (mistaking it for a noisy laptop). Ding!