An Interesting Discovery

[Really Weird Person]

I have made an interesting discovery about Modplug Tracker. It apparently recognizes a three-speaker setup. A panning of zero equals the audio coming out the left speaker only. A panning of 256 equals the audio coming out the right speaker only. A panning of 128 equals the audio coming out the center speaker only. It would seem that it may not recognize higher than this though. I have a 5.1 sound system, but Modplug Tracker does not seem to send much signal to the rear speakers. Feature request: Fade front and/or rear
Concept art

[Saga Musix]

While you have discovered that your 5.1 setup can output stereo sound, you apparently have not discovered that ModPlug has been able to output surround sound for more than 10 years already. Obviously this does not work if you use a stereo output in the setup dialog. And no, OpenMPT does not output sound to 3 speakers in your case, it's your surround system / amp that controls the middle speaker.

[Really Weird Person]

ModPlug has been able to output surround sound for more than 10 years already.

Perhaps that is what the Surround button is for, although I think that I have tried using that and I do not recall it working. Perhaps I am misunderstanding what it does or it does not work properly. If the second one is true, then it has possibly not ever worked properly (since I think that Modplug Tracker is only about ten year old anyway).
Hmmm, I did try checking the Surround button on each channel and enabling the Pro-Logic Surround. The audio still is not as crisp out the rear speakers as it is out the front. For a good comparison, use this game..

[Saga Musix]

since I think that Modplug Tracker is only about ten year old anyway

go educate yourself at the front page. and read the OHM for hints on how to access the rear channels.

[Really Weird Person]

OK, I got surround sound working now. It seems though that Modplug Tracker does not support higher than a quad-speaker setup. In case you are curious about my setup (for purposes of troubleshooting why true 5.1 surround sound is not working for me) (in case Modplug Tracker does indeed support 5.1 surround sound), I have the Sound Blaster X-Fi Titanium sound card and the Logitech X-540 RT standard system.