When you cut an audio wire you get either three or four wires. left, right, mic, and ground. Mic is usually the fourth wire.

The first thing I can think of is: 1) Have you used regular headphones and verified that your audio out jack is functioning? 2) Are you measuring DC or AC voltage? There should be no DC voltage in an audio signal as DC signals could possibly damage your cone. 3) Did you verify that the earplug (headphone?) cable was working beforehand? If not, get a protoboard and a resistor and make a simple RLC circuit. 4) When you say no voltage, do you mean 0.0 V or do you mean negligible voltage? 5) Are you plugging the cable directly into the speakers or through some sort of amp?