I'm not the first person to publish an Instructable for a USB volume control, but I think this one is about as simple and cheap as it gets, and at the same time can be extended at minimal cost to various other functions such as:
and many more, subject to support being provided by your operating system. Mine implements the volume and mute functions (mute by pressing the knob) and the 3 main keyboard LEDs.
Have you ever clicked on a Youtube video and then fumbled for the volume control buttons when it started playing far too loud? Older laptops used to have a physical volume control but this is rare on newer ones and on desktop computers, which is why I wanted to build this.
And how many times have you carried on typing long after accidentally hitting Caps Lock, maybe on a scrabble tile type keyboard with the Caps Lock button a bit too close to the A key, and maybe having an inadequate Caps Lock light or no light at all? That was the other reason.
I assume you'll probably use a different box and so I leave most of the mechanical details to you, Other Instructables may give you a more aesthetically pleasing product using computerised manufacturing but my objective is to concentrate on the electronics.
This project is built around the Arduino Pro Micro (5V version), available from Far Eastern suppliers for £2 - £3. Other ATMega 32u4 based devices could be used, but not ones based on the ATmega328 such as the Nano or Pro Mini.
You also need a 5-pin rotary encoder, which you can probably find for £0.99, and a knob to suit.
The remaining parts you may have lying around: