I bought a Xonar DGX to hook it up to my Sony 5.1 surround system with SPDIF. Windows 10 says the SPDIF port only supports stereo and when I start the sound test it plays only sounds on FL and FR. In the settings of the SPDIF pass through device I can start the Dolby and DTS test, which plays sounds on all 6 channels. When I play AAC or DTS with VLC, only stereo will be played. When I enable "use SPDIF when possible" in VLC settings, it will play DTS in 5.1 and AAC in 2.0. Even video games like Witcher 3 or Dark Souls 3 output only 2 channels.
What do I have to do to always output 5.1 over SPDIF?