Any conductive object can be an antenna. However, effective antennas are approximately 1/4 the wavelength of the radio signal being transmitted, or 1/2 the wave

Any conductive object can be an antenna. However, effective antennas are approximately 1/4 the wavelength of the radio signal being transmitted, or 1/2 the wavelength for a dipole antenna. Headphone wires, approximately 2m in length, could be effective antennas for 40 MHz or 80 MHz radio signals. (80 MHz, you'll note, is close to the 87.5 MHz-108.0 MHz FM radio bands, which might give you a clue why some phones with built-in FM receivers require the headphones to be plugged in). However, music has frequencies in the range of a few dozen hertz to a few kHz, which is in the "ELF" and "VLF" extremely or very low frequency bands. Some communication occurs at these frequencies, but it requires high-power transmitters with antenna arrays that are massive civil engineering projects tens of kilometers long rather than 2m long. These transmit to submarines towing long wires at small, inefficient fractions of the target wavelength, because these frequencies can penetrate saltwater. Also, the headphone cables are not a single wire, they're a pair a pair of wires, each side carrying both a signal and the return or ground loop. You might be able to inductively couple to this loop, like you would inadvertently if you're running unshielded, separated speaker wires near unshielded, separated 60 Hz power cables with poor grounding - but at that point when you're running the receiver wire a few millimeters away from the target's headphone cable, I think they'll notice and object to your tracking attempts...