You can use blocking capacitors (to block the DC) and blocking inductors (to block the AC) - but at the low frequencies you're probably thinking of using?, and the high DC currents involved, it's not going to be simple, small, easy, or cheap!.
Assuming this is for use in a car?, FAR, FAR easier to run a wire!.
I agree with Nigel in general, but if you wanted to send the audio through say the light wiring (eg. the parker lights) then it may be possible since the current is relatively small. Use an inductor (one that can carry the DC current without saturating) at the switch end and blocking capacitors to insert and (at the other end) extract the audio.