Had this strange problem this evening when the sound icon in the menu bar became disabled with a grey icon and didn’t come back even after restarting the machine. After googling for couple of minutes I found this solution and it worked like a charm. Apply the following commands in your terminal

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sudo chown -R _coreaudiod:admin /Library/Preferences/Audio
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The command above will fix the permissions. Now lets fix the dynamic linkers using the following command

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sudo update_dyld_shared_cache -force
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Now restart the coreaudio service

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sudo killall coreaudiod
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Tada! The sound is back 🙂