1) When I press the Power button on my G74SX, the Power icon on the button is illuminated and then Windows starts up. Does the Power icon remain dark on your failed startup attempts OR is it illuminated and Windows just fails to start?
2) If you connect an external monitor or LCD TV to your laptop, does any video appear on it (even only a flashing cursor on a black screen) during a failed startup OR a failed Resume from sleep?
3) If you wait 5 minutes after a failled Resume from sleep leaves you with a black screen, doess pressing Fn-F7 restore video?
4) What version video driver are you currently running?
5) If you go to "Control Panel - Uninstall a program", right-click on "ATK Package" and choose "Repair" from the popup menu, does it resolve either your Power button issue or "black screen on Resume from sleep" issue?
6) If you perform an ASUS Factory Restore (press F9 at the ASUS splash screen and choose to restore Windows to the 1st partition - what they will likely do at the ASUS Service Center), does it resolve your issues?
I think your Power button issue and speaker issue are most likely hardware problems. You may need to bite the bullet and RMA your laptop to ASUS for warranty repair.
G74SX-A1 - stock hardware - BIOS 202 - 2nd Monitor VISIO VF551XVT