Why is the audio is not working after upgrading to Yosemite?
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: