I'm writing a mediaplayer-gui fitting some needs of a medialibrary containing classical music only.
Language is python3/tkinter.
One backend is gstreamer1.0, playbin (seems to be the only one, playing gapless).
When playbin gets the uri of a file with 5.0 channels
(FRONT_LEFT,FRONT_RIGHT,FRONT_CENTER,REAR_LEFT,REAR_RIGHT)
it gives following warning:
** (python3:13745): WARNING **: Unpositioned audio channel position flag set but channel positions present
and plays the file downmixed to stereo.
5.0 is most common in classical-music media(LFE is mostly unwanted).
Which gstreamer-object is the one, i can tell about channel-layout and what signal do i have to connect to, to get that object?
Additional info:
5.1 gives the same warning, but plays without downmixing; 
5.0 using gstplay-1.0 from commandline gives warning & downmixing; 
using gst123 based on gstreamer0.1 plays everything right
I'd suggest to file a bug and ideally make your test files available.
If you want to track this down yourself take a look at the GST_DEBUG="*:3" ./your-app output to see which element is emitting the warning.
how to make playbin of gstreamer1.0 playing multichannel-audio 5.0 playing without downmixing to stereo