Q: no sound

Open System Preferences > Sound > Output

Make sure the correct device is selected.

Is it not playing any sound from various applications?

Youtube, iTunes, Mail, System Preferences?

Is the volume icon greyed out or does it move?

Is it muted?

In the sys prefs under sound > output  = what do you see listed here?

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. Run Software Update and make sure you have a fully up-to-date installation of OS X.

3. If you've installed an application 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 a software 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.

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