The command "mixer /listmidi" works with the win32 midi device only!
What I think is happening here is: The MT32 patch makes MT32 emu the new default midi device (which I don't like really). So if you start up DOSBox with "mididevice=default", it tries to load the MT32 emulator. If you set up your config pointing to the roms in the DOSBox directory and you start up, MT32 emu will successfully be loaded, but "mixer /listmidi" won't show nothing. Now if you remove the roms from the directory, the MT32 emu won't load and DOSBox does a fallback to the win32 midi device. And, as we now know, "mixer /listmidi" works with that one and will list the midi devices successfully again. Please have a look at your DOSBox status windows, I bet it looks like this:
DOSBox_win32_fallback.jpg
As you can see here, DOSbox doesn't care if the roms are in the same directory:
DOSBox_win32_listmidi.jpg
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