Please can someone help me I am so frustrated!!! I am running 2009 macbook pro 17" 10.7.3 and Plex. Last week the sound was working fine (DTS, AC3, stereo), except for some clipping sounds every 30 seconds so I tried to fix this. I tried to change some settings in the audio MIDI settings, but now nothing works at all. NO sound period. I have tried all the combinations to get it to play even stereo but nothing. The optical cable is perfect, and when there is signal it lights up red. I've tried resetting the PRAM, SMC, fixing permissions and booted from a USB drive with a copy of lion and still no sound Please help this is driving me nuts!!!