You mean a method for using a variable rotary potentiometer to adjust amplitude of sound output? There are several such patents (some, I'd imagine, expired by now). There isn't a patent on the concept of "make it louder," but radio manufacturers can't just copy someone else's circuits. They have to either develop their own or buy their potentiometers from the patent holder or a licensee.
>>You mean a method for using a variable rotary potentiometer to adjust amplitude of sound output?<<
Yes, but although different volume knobs work in different ways, the way the human interacts with it is the same. Some volume knobs click at each step and use a completely different signal path for each “click” of the knob. It’s like a steering wheel on a car. You can have rack and pinion, recirculating ball, or even “drive by wire”, all patentable, but the driver is just using the same ol’ steering wheel.