I own a regular Macbook and this problem happened to me a few weeks ago on election day.
I for one think the entire thing is a software issue, something most likely to do with Flash or an iTunes update. My computer I think has some sort of issue with a faulty sound driver (not sure if that's the correct term for Macs)
Issues:
My computer detects no onboard sound card
Movies are having lagging issues with playback. Flash videos online load, play for 3 seconds, then pause. I need to keep jumping to continue the movie. Quicktime / iTunes movies have significant lagtime when trying to play, stop, or skip a movie (and this has NEVER happened before)
International "No" symbol when trying to adjust the volume from the keyboard, missing volume graphic on the finder bar (though the space for it is still there).
Music through iTunes will not play, period.
I'd assume with this happening to so many people, that by now Apple would have a growing awareness of the problem. Guys at the Apple store try to run a system update, find nothing, then say they want to "hold onto the computer" for a few days. As a student, I can't afford to give up my laptop, even for a few days, at least until the semester is over. So now I'm stuck using a computer with no sound / media playback that (at least otherwise) functions normally.
In theory, I have something that I want to try, to see if it's a hardware or software issue: (you need two Macbooks + a Firewire-Firewire connecting cable.)
1) Boot your Macbook in Firewire target disk mode - Hold "T"at startup - this allows you to plug your computer into another computer as a recognizable "hard drive"
2) Connect the two computers (yours in disk mode, the other off) together with a Firewire-Firewire cable, and
3) Boot the other Macbook and hold Option-Command-Shift-Delete, this should Bypass the primary startup volume and seek a different startup volume (such as a CD or external disk)
4) Select your original, now connected, target hard drive to boot from.
In theory, I should be able to run my software off of the other computer's hardware. If there is sound on the other computer, it's a hardware issue on your computer. If there's no sound still, it's a software issue, and it's a bigger problem for you.
At some point i'll try it when I get my hands on a connector cable. If anyone is willing to try it before hand, let me know how it goes.