There were known problems about this with Windows 10 Update (see Method 3).
Method 1:
Start → Run → mmsys.cpl
Enable ‘Disabled’ items e.g. FXSound
Method 2:
Uninstall the sound drivers via the Device Manager (without deletion of drivers files).
Windows will redetect the sound drivers and enable the missing devices…
Method 3:
Run the Command prompt as Administrator.
net localgroup Administrators /add networkservice
net localgroup Administrators /add localservice
exit
Reboot