Seems that the screen lock disconnects the Monitor and reconnects it later. 
This always Switches the Gnome sound output to HDMI / DisplayPort which is not
used in my case. So i always have to go to Settings -> Audio -> click to my
output device which is Digital Output (S/PDIF) in my case. 

Steps to reproduce:

Wait for the screen lock to occur
Unlock the screen.


Expected behavior:

Sound is always selecting the same output as before.


I think this is somehow related to 
https://bugzilla.opensuse.org/show_bug.cgi?id=1195464