This is one annoying issue I Suffered until today, My Friend Sandaruwan advised me to upgrade my Old Ubuntu OS (Ubuntu 6.06) to the New 6.10 version to get out of my sound problem !!! This was some special case in my Acer Laptop.
with some linux flavors the sound drivers did not work hand to hand !!! so I never heard any sound when working in Linux. This was kind of annoying and disappointing for a new Linux User like me !!! anyways I found the solution The new Ubuntu 6.10 is compatible with the Acer On board sound card !!! So any one Suffering from the same prob You only have to simply Upgrade Ur Ubuntu OS .