no audio after QT 7 installation

I installed QuickTime 7 (7.6.6) yesterday as I needed to play an AVI file, which requires certain codec and couldn't run with Perian. The file was played quite well yesterday, but now I don't have sound at all. Just MUTE.

I don't see "built in speakers" in Application\Utilities\Audio and MIDI Configuration. somehow my audio card is just missing. I'm running Macbook Air 10.7.5

The folder System\Library\QuickTime contains the following:

AppleGVAHW.component

ApplePixletVideo.component

AppleProResDecoder.component

AppleVAH264HW.component

QuartzComposer.component

QuickTime3GPP.component

QuickTimeComponents.component

QuickTimeFireWireDV.component

QuickTimeH264.component

QuickTimeIIDCDigitizer.component

QuickTimeImporters.component

QuickTimeMPEG.component

QuickTimeMPEG4.component

QuickTimeStreaming.component

QuickTimeUSBVDCDigitizer.component

QuickTimeVR.component

Is it better to restore my sistem? (which is quite a fuss because there's no internal DVD rom) or is there any way to solve this?

Please help :'(