Have you checked the headphone jack? Often the jack causes a situation where the computer "thinks" there is an optical audio device attached. So plug in a standard headphone.
Have you tried a PRAM and SMC reset? Sometimes that can resolve problems.
Some other possible cures:
Repair Permissions – Repairing permissions via the Disk Utility is something that is easy to do but can take a little bit of time.
Toss the .plist file for the sound app that's acting strange. Deleting a corrupt preference file can "fix" many strange application behaviors.
Reinstall the latest OS X Software Update – this is a great fix for all sorts of issues as well. Go to the Apple site and download the latest Combo Update for your version of OS X. And be sure to Repair Permissions after installing the combo update.
Start in Safe Mode – try booting in “Safe Mode” by holding down the Shift key when starting up. Check to see if the audio is working after that. If it does work it rules out hardware as the cause, there may be some other type of conflict going on and I would continue with the tips below.
Delete the BezelServices preferences file – this file controls those on-screen images that you see when you change the volume, eject DVDs/CDs, change brightness, etc. But it also seems to be the source of all sorts of odd issues that you can never figure out, and tends to get corrupted. Go to ~Library/Preferences/com.apple.BezelServices.plist delete it, log out and in or reboot.
Plug in a USB headset – sometimes your Mac simply gets confused and needs to be literally poked by another audio output device. If you have a USB headset or speaker, plug that in and see if you get audio output through that device. If you do, that’s great! Then try unplugging the USB device and see if your on-board audio output device starts working. (Give it a reboot as well just to be sure it “sticks”).
If it's none of these then you'll have to replace the logic board as there are no sound cards on Apple laptops audio is integrated to the logic board.
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