Some time ago I tried using Mix_SetPosition() to test the idea of using rudimentary audio positioning in my engine. Simply put, wrong channels were used. Tried using both mono and stereo audio files and Mix_SetPanning() with pretty much the same result. After that I decided to make a separate test to see how playback of 5.1 ogg is handled and I noticed the same thing. Same file was played correctly in various media players and other applications. Results look like this (with test app restarted between tests):

Speakers set to 2.0/2.1:
src    SDL     should be
FL     FL      FL
FC     FR      FL/FR
FR     FL/FR   FR
BL     ?       FL
BR     FL      FR
LF     LF      LF

Speakers set to 4.0/4.1:
src    SDL     should be
FL     FL      FL
FC     FR      FL/FR
FR     FL/FR   FR
BL     ?       BL
BR     BL      BR
LF     LF      LF

Speakers set to 5.1:
src    SDL    OK
FL     FL     FL
FC     FR     FC
FR     FC     FR
BL     ?      BL
BR     BL     BR
LF     LF     LF

This test was performed using:
- SDL 2.0.8 (x86)
- pre-release SDL_mixer 2.0.3 (x86) "released" on discourse 
(https://discourse.libsdl.org/t/sdl-mixer-2-0-3-prerelease/23959)
- Windows 7 64-bit
- Auzentech X-Fi Prelude 7.1 sound card
- 5.1 speaker set
- 5.1 ogg file from 
https://people.xiph.org/~xiphmont/demo/surround/demo.html - for some reason SDL_mixer refused to play 5.1 wav file

Had no chance to test it in SDL_mixer 2.0.4, but I haven't seen any mention of it in changelog, so I assume it's still the case.