After fumbling with Scart, HDMI-ARC, some digital converter to Toslink (with and without Scart-adapter), I’ve finally managed to use my Sony 5.1 surround sound system:

https://i.imgur.com/ey7Cfbw.jpg

Took me just an external audio card and some patience to re-install the driver from the CD and to find the right way to plug everything in and to set things up