Have you ever wanted to boot to recovery or safe mode?
Target Disk Mode
Option + T
Command Line Mode
Command + S
type fsck -cy
Recovery Mode or Selectable Boot Mode
Command + R or Option
Hold Shift after startup tone on boot
Release when you see the Apple in the center of the screen
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Have you ever unplugged your headphones, and the internal speaker does not work, and it is simply greyed out? You will also see a red light coming out of the headphone jack.
Use a toothpick and wiggle it in the headphone jack hole. You can also insert and remove the headphones a few times and the internal speaker should kick on.
Sees to be a sensor gets stuck that turns on the Optical Input inside the headphone jack.