This is a crude animation of the middle ear action. Sound hits the (red) eardrum, moves the three little bones, and pushes on the fluid in the cochlea. The (beigish) muscle coming out into the foreground is the Tensor Tympani. It attaches onto the other side of the middle ear chamber. When it tenses, it sort of 'jams' the bearings of the ossicles, making movement harder.
Normally the Tensor Tympani acts involuntarily when a loud sound is perceived. But all involuntary muscles can be controlled voluntarily with sufficient practice.
Voluntary muscles are equipped with a beautifully complete set of built-in feedback sensors for bend, stretch, and acceleration. We know without any special mental effort when a finger is bent halfway. Involuntary muscles lack those sensors, so you have to find an indirect feedback indication and then screw around with various 'efforts' until you pick up the feedback.
For a nearby example, I learned a long time ago how to open my Eustachian tubes to pop each ear, without any visible action like yawning or chewing. The pop was the feedback, and I just kept 'throwing' effort into various 'places' until the pop happened. Now I can do it on command, almost as precisely as bending a finger.
Many people are able to control vasodilator muscles by monitoring the sense of warmth in the extremity when the arteries are open.
For the TT, the obvious signal would be a damping of steady sound input. Another possible learning path: The TT always tenses when you talk, because your own voice in such close quarters can be strong enough to damage your cochleas. Sometimes when I'm tired or sleepless, with inhibitory mechanisms presumably weakened, I can actually hear the TT acting with a 'thump' when I vocalize.
If you can master tightening and holding the TT, you'll have earlids.
The current icon shows Polistra using a Personal Equation Machine.