I don't think there's a way to make it stay on audio permanently, but...as long as you have it switched to that, it should stay that way unless it's switched back to video.

[He looks down at his own Gear as he talks, running his fingers along the face so he can feel the buttons. There are buttons for pretty much everything here, as well as a switch for shifting between video and audio.]

There should be a switch on the...right side of it. Yours is on video right now, so it's probably pushed down. If you push it up, it will turn off and go to Audio only instead.

[He knows there are probably internal settings that could potentially make more permanent changes like that, but honestly he's never really messed with them too much.]