I have even put a headset in and that dosen't work either! I'm trying to use garageband as i need for my job and i seriously need help!
Try checking your setting.
Go to system preference -> speech -> speech recognition and calibrate the internal microphone
Go to system preference -> sound -> input, at the input volume see if the slider is at the left, adjust the slider. ( i usually put my in the middle incase something get too loud)
If that doesn't work, try resetting your PMU.
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