To do this, you need to bias the output signal so that ground is about 2.5 volts higher than normal, so-as to pull the signal down and center it. Then, you use a small trimmer potentiometer to reduce the signal (via a voltage divider).
Or more simply wire a series 1mfd capacitor from the pin to the pot. That will remove the DC bias voltage, leaving only AC audio frequencies to pass. As already stated, you can't directly generate sine wave audio, just square wave that have lots of odd harmonic frequencies, so it will always sound buzzy and harsh.
Lefty