As of 24/08/2012 sound is enabled by default in the Raspian Distribution, the following pay applies to the Debian Squeeze build which is also commonly used.
Getting Sound Going
install sound utilies
sudo apt-get install alsa-utils
load the sound driver
sudo modprobe snd_bcm2835
sudo aplay /usr/share/sounds/alsa/Front_Center.wav
permanently load the sound driver
to load the sound driver every boot add it to config file
sudo nano /etc/modules
snd_bcm2835 to the end of the file.
The sound driver will be loaded at boot time.