Let's make it easier for sighted users (who may be unfamiliar with how ChromeVox works) to disable ChromeVox. 

1) Make the tap target for the X close button on the caption panel larger. With this, consider making the top of the screen a bit less sensitive to volume adjustment. I just tried to tap the X button and missed slightly, which instead adjusted the volume to 100%. 

2) Add a label to that button so that when focused, the panel reads "Disable ChromeVox, button - double tap to select" to give less familiar users a tip on how to activate the button.