1) Go through setup etc.., log in to Windows 8.1.
2) Reboot, this is important, so that your Audio device is detected.
3) Control Panel > Sound > Click on your Audio device, mine is connected to Yamamha AMP, shows as Panasonic TV connected to AMP.
4) Click on 'Set as Default'.
5) Click on Apply.
Thats its, should be remembered now.