Okay,

There are several ways to control the volume of a stereo system, but none I can think of using a computer.

However,

The volume knob on a radio is usually just a potentiometer (variable resistor). What you could do is find the wires for the resistor and add either a switch or another resistor. The higher the value of the resistor the quieter it will be. Adding another potentiometer would just add to the personal fun. The only problem is you would need a small hole through the wall for the wires for you to adjust the potentiometer.

Another thing you could do is use a couple of IR boosters. There are a couple of models used for car audio that are fairly small, and you could even feasably use a universal remote co control it through the wall.

These are just suggestions in the event you want to gaslight your brother. Post the Make and Model number in the event you want me to make a diagram.

I just love practical jokes!