Q: Suddenly lost all sound from internal speakers.

This problem was corrected by turning off all muting in the Audio MIDI Setup utility in the Utilities applications folder.  I had never heard of the MIDI Setup until doing a Google search yesterday.  I have no idea how muting got turned on in the MIDI Setup since I had never been there.

There are many possible causes for this issue. Take each of the following steps that you haven't already tried.

1. Start with the steps recommended in this support article. Don't skip any of the steps. It's the starting point for further efforts to solve the problem.

2. If your model has an audio-out (headphone) port, and anything is plugged in, unplug it before putting the system to sleep.

3. If you've installed something called "Memory Clean" or any other third-party software that is supposed to "clean" or "purge" memory automatically, remove it according to the developer's instructions and restart. You should do that even if the software is not causing the problem, because it's useless.

4. If you've installed an equalizer called "Boom" or anything similar, update or delete it and restart.

5. If an AirPlay device is selected for sound output from iTunes (or from other applications via third-party software such as "Airfoil"), deselect it.

6. Launch the application "Audio MIDI Setup" by entering the first few letters of its name in a Spotlight search and selecting it in the results (it should be at the top.) Select the Output tab. If the Mute boxes are checked, uncheck them. Move the Volume sliders all the way to the right.

7. If a red light is coming from the audio-out port, the internal switch is stuck in the position for digital output. You may be able to free it by inserting and removing a mini-stereo jack of the proper size. Inserting any kind of tool in the port may cause damage that won't be covered by your warranty.

If there's no red light, the switch may still be stuck in the headphone position. Try to free it the same way.

8. Disconnect all wired peripherals except keyboard and mouse, if applicable. If more than one display is connected, disconnect all extra ones. Restart and test.

9. If you have a MacBook Air, turn off Bluetooth and restart.

10. Start up in safe mode. Don't log in; just restart as usual when the login screen appears. When you do, make sure the words "Safe Boot" do not appear in the login screen. If they do, the system is still in safe mode and sound won't work.

11. Reset the NVRAM.

12. Reset the SMC.

13. Reinstall OS X after backing up all data. For a possible shortcut, see this thread.

14. Make a "Genius" appointment at an Apple Store.

I still have this problem too. It started for me on Mountain Lion. It was actually sitting right in front of me playing sound when the sound suddenly quit mid-song. Tried everything - PRAM, SMC, upgrading to 10.9.1.

Is there any chance that I *won't* have to make an 8 hour round trip to Chengdu to visit an Apple Store?

Awesome post, thank you. The red light in my Audio Out port was on, and poking it with a headphone plug a few times fixed it!