After nearly a year of everything working properly, my 27" LED Cinema Display's SOUND suddenly stops working!! I searched for a fix on the internet but the only thing i can find is something about a firmware update that fixes a sound issue. This was released a long time ago though and i don't know what version mine is??? How do I check the version of my display? Also, how do I update it? Would it have been updated automatically via Software Update?
If it is not in your software update you probably don't need it.
If you explicitly download it and try to install it it won't install if it doesn't think it has to.
Just because no sound comes out of the display doesn't mean its a firmware issue. More likely its a USB or software issue. Particularly if it worked for a year. Firmware doesn't "bit rot". Software, on the other hand can be corrupted by other software (e.g., plists), and USB's can be "moody" at times (audio is on the USB - is your display usb plugged in?). They also only have so much power to go around for all the devices on them which is why some people need powered usb hubs.
First I would check your sound system preferences to make sure the volume is "up", not muted, and you are outputting to the display audio.
Try doing a smc reset.
See if you can audio out of the internal speaker and/or stereo headphone jack.