This can occur if the personality card in the computer has become unseated. Once the personality card is reseated properly (following all appropriate safety precautions), the error should go away and the system sound should operate properly. If you hear no sound after reseating the card, try these things (in order):
- Check all external speakers/headphones as possible sources of the problem.
- Check the speaker connection to the logic board. (An Apple Service Provider may need to do this for you.)
- Replace the speaker. (An Apple Service Provider will likely have to do this for you.)