I posted about this in General:
http://www.raspberrypi.org/phpBB3/viewt ... 72#p276172
But I've tried a few more things now and I'm hoping somebody else might have solved the same problem.
Basically, sound is distorted playing through the 3.5mm audio jack, particularly at low volumes. The sound is like this with a clean Raspbian installation (2012-12-16). I'm using amixer to turn down the volume (sudo amixer set PCM -- -5000) and aplay on some of the preinstalled wav files in /usr/share/scratch/Media/Sounds/. I've also tried doing "sudo amixer cset numid=3 1" to no effect.
I have been using the RPi to play through external speakers with mpd, but the distortion is less obvious with software volume turned up to 100%, so I can't tell whether this problem has always been there and I didn't notice or is new. I have moved the RPi around a few times plugged into speakers and I wondered if I damaged it.
I'm going to try buying an external USB sound card to see if this addresses the problem. Any other suggestions?