Distorted audio in Musescore 1.3 on Mountain Lion OS X 10.8.2

Sadly, V1.3 has not removed the distorted audio for me; it has introduced it! I never had any problems with distorted audio in any version before this one. Now there is a contant chatter in the background, well, almost as loud as the audio, that also seems to be interfering with timing of playback.

I didn't have the install problems reported elsewhere, but I've re-installed with the latest release anyway. Still the same.

I think it might still be related to reverb. I've tured reverb down to zero. It doesn't fix it, but if I enter or sound a single note there is a "slap" immediately after the note sounds. The slap period is the same regardless of the length of the note entered. However when played, on a long note there is a repeated slap throughout the note.

I have tried varios sf2 soundfonts. They are all the same although the slap volume seems to be relative to the volume of the soundfont (some soundfonts seem to be at a higher volume than others).