Would there be some kind of MIDI loopback driver I could use to force this? Sonar does see the Microsoft Wavetable GS driver.

Does Putzlowitsch's see it?

Yes. Everything sees it but my DAW. I guess the changing point might have been when I upgraded to Win10. Still, why only Sonar and not anything else?

So I looked up some loopback drivers and found one that works called LoopB1. Solved the problem for the time being. Still a bit of a nuisance and a puzzle why it won't show up on its own like it used to. I'm not sure what the latency is like either as I could only test on my PC remotely from work (the sound of which is delayed anyway). But it's better than nothing.

I would suggest LoopMIDI instead.

Although I can't think of a reason why the Munt driver wouldn't appear in Sonar, except maybe if you have too many synths in your MIDI list.

That's never been a problem before. I've also had an OPL MIDI synth driver that showed up along with MUNT, my two wavetable synths, Microsoft GS, and two external MIDI outputs. Incidentally, the OPL3 one doesn't show up either now for some reason. Both since updating to Win10. And, of course, it detected LoopB1 just fine.

Any particular reason to use LoopMIDI over LoopB1?

I used LoopBe1 a few years ago, but had constant issues with sysex transmissions. I switched to LoopMIDI which has several advantages, like hotplug MIDI port creation and AppleMIDI support.

Is your MIDI sequencer x64 application? x64 apps can't use 32bit user mode MIDI drivers(like MUNT, OPL3). LoopMIDI is kernel mode driver, so it works with both 64 and 32 bit apps. You either should switch to 32 bit version of your app or build x64 version of MUNT.

That's it. That makes perfect sense. Don't know why I didn't think of that. Thank you! Strange that the loopback driver does work, though.

Is there any advantage to using an x64 sequencer?

When you're dealing with massive sound libraries, quality VSTs, and dozens of recorded audio tracks with layered real-time effects, yes there's a benefit. I dare not run 32-bit anymore. Need the memory.