I use JRiver Media Center 20.0.80 on a Windows 7 x64 machine.
For audio playback I use a Nubert AS-250 soundbar which is conntected as an USB DAC.
In JRiver music options I choose USB AUDIO DAC [WASAPI].
When I play a movie with DTS Audio codec 5.1 (3F2R/LFE) I tried the DSP options 2 channels (stereo) with JRSS and without mixing.
In both cases I can hardly hear any voices, but just background effects. Unfortunately I could not find any settings that made the movie audio output sound normal.
When I open the movie in VLC the audio is just fine and I can hear the voices normally and it seems to be the right audio balance.
Does anybody know which settings I have to use to get a good 5.1 to 2.0 downmix that I can enjoy movies also with JRiver?
If you have the output format settings set to "2 channel (stereo)" with JRSS mixing enabled you should get the correct downmix. You have to stop and restart playback for some DSP options to take effect, so make sure to try making the DSP changes with playback stopped.
Also you might want to make sure that you don't have output encoding or bitstreaming enabled.
If you have the output format settings set correctly and you're still not getting a correct downmix, during video playback try clicking on the audio path icon and post what it says there (the icon is three faders, located near the upper right corner, but below and to the left of the search box)