With '52, it is simple: connect T2EX pin to your audio input and program Timer 2 to generate required frequency. HOW? It is in the spec...
However, there may be some problems, as your frequency will be derived from quartz frequency, fCLK/N, while N is *integer* value. Only some frequencies will be available.

For how to generate a long pulse (several seconds), look at older messages, I think, I saw an example already. Using only another timer in 16-bit mode won't enough, since maximum duration will be too small: about 70 ms @ 12 MHz quartz.