OS: 64-bit Windows 10 21H1
Version: Doom Legacy 1.48.10
Sometimes, when I play Doom Legacy, it plays the MIDIs on my Roland Sound Canvas SC-55ST unit, as it should.
But sometimes, even within the same game session, it instead plays the currently playing MIDI on Microsoft' GS Wavetable Synth, even though I have Coolsoft MIDI Mapper set to map MIDI to my Roland SC-55ST unit.
I understand that not everyone has a Roland Sound Canvas SC-55ST unit (or really any MIDI module) and a USB-to-MIDI cable to hook it up to.
However, most modern Doom source ports like PrBoom+, Chocolate Doom, Crispy Doom, RUDE, LZDoom, GZDoom, and even most older Doom source ports like Boom, ZDoom, Doom95, and MBF don't have this issue.
The steering of music to a particular player is done by SDL, using that particular OS, and maybe some other actors. I currently do not know if there are any controls over this, but for several reasons will be looking into trying to get some control over this.