no sound

I have also lost sound output on my Imac. This started since my upgrade to 10.7.4. If I reboot then the sound returns until I open google chrome or Safari. Checked preferences and not showing Mute and my internal speakers show as the default. Driving me mad.

Any help please?

Your 'Airplay ' or 'Apple TV' mode is on. Shut it off and switch back to iPad mode.

SMC RESET

1. Shut down the computer.
2. Unplug the computer's power cord and all peripherals.
3. Press and hold the power button for 5 seconds.
4. Release the power button.
5. Attach the computers power cable.
6. Press the power button to turn on the computer.

PRAM RESET

1. Shut down the computer.
2. Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
3. Turn on the computer.
4. Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
6. Release the keys.