I recently installed Ubuntu 10.04 on my Mac mini and sound did not work at first. The Mac mini has a small internal speaker that works fine under Mac Os X, but I haven’t gotten it working under Ubuntu. I did get sound working through the headphone jack, though by following the instructions here. Don’t forget to reboot.
Edit: I upgraded (It’s an automated process, but it took hours.) to Ubuntu 10.10 Maverick. I had to edit the configuration file again using the instructions mentioned above. After I rebooted the little internal speaker worked. When I plug in my headphones it cuts out the speaker and sends the music, appropriately, to the headphones, but there seems to be more hiss or buzz when no music is playing.