thats what the manual says

Multichannel
PowerDVD and WinDVD can also operate as software decoder, sending a DVD's multichannel
data stream directly to the analog outputs of the Fireface. For this to work select the WDM playback device ’Loudspeaker’ of the Fireface in >Control Panel/ Sound/ Playback < as Default.
Additionally the loudspeaker setup, found under Configuration, has to be changed from Stereo
to 5.1 Surround.
PowerDVD's and WinDVD's audio properties now list several multichannel modes. If one of
these is selected, the software sends the decoded analog multichannel data to the Fireface.
TotalMix can then be used to play back via any desired output channels.
The typical channel assignment for surround playback is:
1 - Left
2 - Right
3 - Center
4 - LFE (Low Frequency Effects)
5 - SL (Surround Left)
6 - SR (Surround Right)
Note 1: Selecting the Fireface to

when i plug the 3 output (center) and the 4 output (LFE) i get sound only on channels 1,2,5,6. how do i enable the center and sub?
do i need to set something in totalmix?

if it cant be working in RME, can Voicemeter help?

please try to help
thank you!