I have a fairly new Toshiba Satellite U945 ultrabook and I just noticed all of a sudden that the audio icon has a red x indicating that there is no sound.  It also indicates that there is no audio output device indicated.  What happened? I tried reinstalling the Realtek High Definition Audio software, but still does not work. Any advice on how to go about this?

You can try to restore your PC the time when it worked.

http://windows.microsoft.com/en-us/windows-8/restore-refresh-reset-pc

Hi,

Please open Device Manager to see if there's a yellow exclamation mark on Audio Device.

If so,please update the latest version of Audio driver from manufacturer website.

Also,check your sound volume to see if it was set to mute.

Regards,