A plague on all your houses core audio. A plague I say!
My recent system update that pushed me to OS 10.7.4 brought with it an array of issues connected to the system process CoreAudio. It began with no longer registering the presence of speakers when woken from sleep, and evolved into not every recognizing the internal hardware. After reseting my PRAM, repairing permissions, clearing log files, system maintenance, plist editing, and a litany of other maneuvers, it was resetting the SMC (System Management Controller) that finally did the trick.
If you also happen to be experiencing this same issue – start up chime, but no system sound – this might also be the solution for you.
You can see Apple’s Support Article here: http://support.apple.com/kb/HT3964