I have a desktop computer(HP Intel Core 2 Duo). After installing Ubuntu 9.10, I have no sound.
As a first step, make sure you have set the volume higher than 0%.
If you upgraded Ubuntu and it worked on 9.04 but not anymore in 9.10 check that you are booting the right kernel: vmlinuz-2.6.31-(something) and not the one from previous version vmlinuz-2.6.28-(something).
Second step is to kick the pulseaudio dæmon: