You could use a micro-controller (must have serial I/Os and an ADC), that communicates to a program on the PC using the serial port. This should be easy as most mcus have dedicated pins for serial communication.
You can connect a pot (say 1 K ohm) to the analogue port of the mcu. So that varying the pot varies the analogue voltage given to the ADC port of the mcu.
When the ADC channel voltage is varied, the mcu sends a command to the PC application, the PC application could increase or decrease the audio volume using API calls.
An application in VB could be quite easily made that controls the audio settings and also communicates serially.
thanks
ab