After upgrading my Macbook Pro to Yosemite, the volume icon at the top right is greyed out and sound does not work.
How can I fix this issue?
This can be caused by the default sound settings being changed. If you go to System Preferences --> Sound --> "Sound Effects" you will see a pull down list labeled Play sound effects through... On my machine there was the following listed: Internal Speakers and my monitor.
For some reason after the OS upgrade my monitor (and HDMI) was selected instead of internal speakers. Switching to Internal Speakers fixed this. You should select the appropriate item for your setup. Also, you can go to the Output and Input tabs and verify that the correct item is selected as well. See this picture for additional information:
I had to open Sound Preferences, change to Output tab, and select "Internal Speakers".
I had the same problem. It turned out that I had my external screen plugged in while I ran the update so the sound was defaulting to HDMI out rather than internal sound or bluetooth.
Went to sound preferences, changed it and it worked.
Same problem here, disabled sound control. According to this post you can run
sudo killall coreaudiod
in Terminal for a temporary fix.