You can only use the built in voice recognition to trigger standard P3d commands, so controlling custom gauges would be a no go I think. You may be able to do it with additional software like "voice attack" if you can assign key presses to the functions though.
Took a while to get working. Seems a bit finnicky with the settings but now I am happily shouting at ATC and being told to go around quite a lot
It's also handy for things like unlocking the tail wheel.
The default command "Make it snow" made me laugth.
ATB,
Ted.