The MacBook Pro internal speakers stopped working
Why would my internal macbook pro speakers stop working but when I connect an external speaker, the external speaker works
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Just make sure the Sound System Preferences/Settings has the "Output" selected for your internal speakers. On some older laptops, the headphone jack can fail due to the one internal contact bending & shorting out against the grounded jack casing...this makes the laptop think headphones or external speakers are connected. When playing audio, look into the headphone jack port...if you see a red light, then the jack is bad. It may be possible to adjust this bent contact, but it is tricky to do since it is so hard to see & work in the port. You may be able to insert & remove the headphones/speaker cable and the contact may move slightly to stop shorting out against the grounded casing.
Otherwise, most likely the internal speakers are bad (or at least the left one which can prevent the right one from working -- crazy I know).