Also a site with notes on how to install and get sound going on raspian. you need to install alsa and a driver to do so.
copied below for reference:
With the Pi connected to the Internet and SSH'ed in (see our previous tutorial) install the alsa audio drivers and MP3 Player
- $ sudo apt-get install alsa-utils
- $ sudo apt-get install mpg321
Reboot the Pi (% reboot) and when it comes back up, load Sound Drivers and Setup for 3.5mm Jack Output
- $ sudo modprobe snd-bcm2835
- $ sudo amixer cset numid=3 1