Doesn't feel like the button is stuck or anything does it? If not, then you've ruled out wiring and physical.
You've also ruled out receiving software, so then it would have to be how the factory system is handling the input. You could try scanning for codes or pulling the battery for 5 mins.
Dude! My steering wheel voice button does not work either... I bought the car used and that button never worked for me, so I always just assumed I was doing something wrong. I'm connected to Car Play with my Android without issue, and the touch screen voice button works fine just like yours.