I've never had any luck getting DOSBox to work with my MT-32. Could it possibly be because the IRQ port isn't set to 9? Do you have to modify anything in the config file?
In the config file of Dosbox, have the following configuration for your MPU-401 emulation:
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[midi]
# mpu401 -- Type of MPU-401 to emulate: none, uart or intelligent.
# device -- Device that will receive the MIDI data from MPU-401.
# This can be default,alsa,oss,win32,coreaudio,none.
# config -- Special configuration options for the device. In Windows put
# the id of the device you want to use. See README for details.
mpu401=intelligent
device=default
config=
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Then make sure that in Windows you have selected your "MPU-401" device or whatever it's called as your default Midi-Player. You can find that screen somewhere in your configuration screen. Not sure how to guide you to that screen, because I use a localized Dutch version of windows. If this setup doesn't help, you could always change "device=default" into "device=win32", they both work for me. Hope you'll be able to use your MT-32 with Dosbox in this way.