Sounds like you're trying to convert the audio signal directly, but all you're doing is grabbing
the audio [sine]waves at different points. 500-1023 indicates you have a nice big signal.
I think what you want is a leaky peak detector. Output of the opAmp goes to a diode anode,
whose cathode then connects to an R in parallel with C. Try maybe 10K and 1-10 uF. Eg,
