In Ubuntu 12.04, after performing an upgrade that included a kernel update, I no longer have any sound. The sound worked fine before the upgrade. Oddly enough, if I plug in headphones I do get sound through the headphones, but without them I am no longer able to get any sound from my laptop's internal speakers. Please let me know if there is anything else I should add.
Please provide what soundcard you have, or what modules are loaded. If unsure how to do that, use this command:
If you have an Intel HDA card, you might try the steps on this page: https://wiki.ubuntu.com/Audio/UpgradingAlsa/DKMS