The headlight line cannot be used - it is a digital input. Ditto for the mute line. If you were electronically competant then you could build a circuit to encode the voltage as a PWM signal and then measure that on the empeg. You'd need to hack the kernel.

The microphone input is _probably_ ac-coupled. So you'd need to modulate the dc voltage onto an ac waveform in order to measure it. Again, you'd need to hack the kernel. You'd lose the use of visuals in Tuner and Aux In modes.

A third solution is to digitise the voltage, and encode it into a serial RS232 message. You could write a user-land application to listen to the messages and adjust the volume.

A fourth solution is to buy a low cost GPS receiver and hack the empeg GPS software to ask for the GPS calculated speed.