Crystal radio as user interface problem
Resistance is a good solution, it also might be interesting to treat the coil like a resonator.

Try using a uC to give it a rising edge on the end clipped into and measure the rising edge on the other side. I could simulate this and see if it was feasible, but it would probably save hours to use a function generator and an Oscope, or just use a uC. A capture pin on a PIC and a PWM output would be my choice, pin goes high from PWM on a PIC at timer overflow, capture is relative to timers.

Just an Idea, if I think of any others I will let you know. If it could be done you could have two simple interrupts and just w check the variable they wrote to before doing any audio out.
-Max