A PIC can only deal with digital data, but that doesn't mean you can't feed it an analog signal. As long as you don't violate the I/O pin's max and min voltage and the signal contains only a single tone you can read it directly from a digital I/O pin. A small signal diode can cut the negative part of the signal off and a zener can limit the maximum voltage to protect the I/O line. Ground one side of the headphone to the ADC ground, use a diode to short the negative half of the audio to ground, then your ADC will only read the positive portion of the signal, which is fine for tone detection.