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Same here and I always sort of assumed this was a normal thing of the human body. I'm kinda shocked to learn that many people can't do it.

What kind of muscle can switch between voluntary or involuntary depending on the person?

As the article mentions, the tensor tympani muscles are also involved in hyperacusis, which is an inability to tolerate sound at volumes most people have no issue with.

One of my kids has this. The best analogy I've found is it is like standing out in the sun when you already have sunburn.

Oh, so that's what that is? Crazy. Subjectively (besides the sound) it just felt like a vague pressure in my head, near the neck, so I never could figure out what the hell it was I was doing.