I have a SheevaPlug (not DreamPlug) and these are the notes of how I got audio working (on Debian Squeeze).
Not much, but it might be worth trying these...
In order to get sound working I had to:
a) Run, as root:
alsactl init 1
The USB sound device ends up as device 1, even though a lot of things
seem to default to using 0.
b) Run, as root:
service alsa-utils start
This was *not* needed after reboots.
c) Add myself to the audio group, as the devices get set to that group