my sound icon has a cross through it with no sound?

Resetting the SMC generally cures it:

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

http://support.apple.com/kb/ht3964

Resetting your Mac's PRAM and NVRAM:

http://support.apple.com/kb/ht1379

Your audio settings may have been changed.

Open Audio MIDI Setup (/Applications/Utilities/), then check the Audio Output setting.

Change the Audio Output setting to 44100.0 Hz.

Quit Audio MIDI Setup.

More info here:

http://support.apple.com/kb/TS1574?viewlocale=en_US

For anyone else with new macs,

Just go to your sound settings, here

and fiddle with the volums controls there,

It worked for me!

Hope it helps,

Lily

This all happened just after I was using some plug-in external speakers.:

I had the "No speaker" symbol when I hit the F speaker volume keys, and no sound. Tried PRAM reset, SMC reset, Repair Premissions. Nothing.

Went to Apple>System Settings>Sound> Output tab. Noticed that under Select a device..>Name, it said something like Optical output (as well as Apple TV and Airport Express below that).

Could not change this setting. Though I remembered it saying Internal Speakers in the past.

Plugged in headphones and computer paused a long time. Beachball city.

Finally got headphones working, and now under Name it said Headphones.

Unplugged Headphones and, after another long pause, "Internal Speakers" reappeared, and all works now.

So, I don't know what happened. Just posting for those who may be having similar problems. Faulty headphone jack? I did see the red light coming from the jack intermittently. Optical?

MBP 2011, running Yosemite, fully updated OS.

I have the same issue as JWD88. My MacBook was trying to do optical. With optical audio, there is no control over the volume from the transmitter (because you are literally sending a light beam). I could see the red light coming from the jack when audio was playing. I hadn't plugged in a optical device, but I was messing around with inputs in Garage Band...

If I plug in headphones, the audio output switches to the built-in output. When I unplug the headphones, it switches back to optical. I ended up having to switch to another output (AirPlay), and then optical audio switched off, and everything started to work properly.

I didn't even know I had optical... 😀 Anyone know how to turn it on/off correctly? Is it supposed to be automatic, and just detect when there is a optical audio cable plugged in? If so, that is broken...

MBP 2011, running Sierra 10.12

Just plug in something with a regular 3.5 mm jack, like headphones or even just a cord. It will switch it back. I'm not sure there is another way to get it to switch off optical.