The phrase "volume knob" refers to a button or dial that adjusts the loudness of sound, like when you turn up or down the noise coming from a speaker or a device such as a radio or a TV. Full definition
Thankfully there's a backup volume knob on the steering wheel. (autofocus.ca)
Again a lack of volume knob means you're better off working with the steering wheel controls. (cnet.com)
The screen can be touched when the vehicle is not moving, or when in motion, it can be controlled by the easy to use rotary knob and volume knob in the console. (bestride.com)