Q: no sound!

Try these in order testing your system after each to see if it's back to normal:

1. a. Resetting your Mac's PRAM and NVRAM

    b. Intel-based Macs: Resetting the System Management Controller (SMC)

2. Restart the computer in Safe Mode, then restart again, normally. If this doesn't help, then:

     Boot to the Recovery HD: Restart the computer and after the chime press and hold down the

     COMMAND and R keys until the Utilities menu screen appears. Alternatively, restart the

     computer and after the chime press and hold down the OPTION key until the boot manager

     screen appears. Select the Recovery HD and click on the downward pointing arrow button.

3. Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the Utilities menu. Repair the Hard Drive and Permissions as follows.

When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list.  In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive.  If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu. Select Restart from the Apple menu.

     Reinstall the 10.9.4 update: OS X Mavericks 10.9.4 Update (Combo).

4. Reinstall Lion/Mountain Lion, Mavericks: Reboot from the Recovery HD. Select Reinstall Lion/Mountain Lion, Mavericks from the Utilities menu, and click on the Continue button.

Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.

     Reinstall the 10.9.4 update: OS X Mavericks 10.9.4 Update (Combo).

There are many possible causes for this issue. Take each of the following steps that you haven't already tried. Some may not apply to your model. Back up all data before making any changes.

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. Other third-party software that has been reported to interfere with sound output includes "Boom" and "Samsung Kies." If applicable, uninstall according to the developer's instructions (not by dragging something to the Trash.)

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 Built-in Output from the list on the left. If the Mute boxes are checked, uncheck them.

7. If a red light is visible in the audio-out port when sound should be playing, 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. Triple-click anywhere in the line below on this page to select it:

/Library/Preferences/Audio

Right-click or control-click the highlighted line and select

          Services ▹ Reveal in Finder (or just Reveal)

from the contextual menu.* A folder should open with an item selected. Move the selected item to the Trash. You may be prompted for your administrator login password. Restart the computer and empty the Trash. Recreate your settings in the Sound preference pane.

*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination  command-C. In the Finder, select

          Go ▹ Go to Folder...

from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.

14. Reinstall OS X after backing up all data.

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

I would reinstall the OS X Mavericks 10.9.4 Update (Combo) that you can download from Apple by clicking on the link.