Try this fix from MIcrosoft.
Speakers not working Windows 8.1 upgrade.
1. Right click speaker icon (right hand corner)
2. Select Playback devices
3. Right click Speakers
4. Enable it and set as Default
5. Click Apply/Ok
Is it on everything or just certain things? Try a music CD does it play fine? What a bout trying a movie?
Please let me know.
Thanks.