Apple has released a firmware update for the 27-inch LED Cinema Display. The 957KB download "addresses intermittent audio issues some users have experienced while using the 27-inch LED Cinema Display." To install the update you must have Mac OS X 10.6.4 or later, plus the confusingly similarly-named LED Cinema Display Software Update 1.0 (released in September). Alternatively, an installation of Mac OS X 10.6.5 will do, as the previous display update was bundled with the 10.6.5 updater.
If you own the latest 27-inch Cinema Display, you'll likely be able to get the new firmware via Software Update; if it's not showing up, it's also available via direct download.