Many are currently having a problem with Realtek HD audio not working after resuming from sleep in Windows 8. The current fix of re-booting is a pain but I found a reliable way to get it working without a reboot
There is an argument between Microsux and Realtek at the moment over who is to blame so I don’t see a fix coming very soon. From this link
“Bug report submitted to Realtek, and they have replied stating that the problem is with Windows 8, since the driver works fine on previous versions of Windows.”
1) Right click the speaker icon in systray (desktop mode) and select ‘Playback Devices’
2) Right click the ‘Sound’ window and make sure ‘Show Disabled Devices’ is ticked.
3) Right click the ‘Speakers’ entry in the list of devices, should have ‘Realtek High Definition Audio’ and also show green tick as default playback device.
4) Select ‘Disable’
5) Right click it again and then enable.
6) If sound is still not working unplug your speakers from the audio out socket of your computer and then plug back in again.
7) You should have sound. Your done.
You will probably need to restart the media player or restart the browser, whatever your using, before audio will work for those programs.
[update] To solve this completely change your desktop power settings to prevent the system going to sleep at all. [/update]