That reminds me—in elementary school, I used to figure out what someone was writing by listening to the sound of their pencil, sometimes by putting my head to the desk. It probably worked mainly because they were writing pretty slowly in large print, but I imagine you could come up with a software implementation as a useless gimmick if you had enough training data.

I’ve considered implementing this as a keyboard layout in Android, so you could put your phone on the desk and type by scratching a stylus or fingernail on the surface next to it.