Transmitting audio over variable voltage DC power line?
You would need to use a modulated signal, not the baseband audio because the low frequencies will be heavily attenuated by capacitance on the d.c. voltage, and because of the rate of change of your d.c. voltage could be at low audio frequencies (rumble). I'm sure you could get reasonable audio quality if you use a suitable modulation that could be seperated out from the d.c./low frequency signal.