try fiddling with the preference file (~/library/preferences/Dosbox 0.74 Preferences).
Look at the [midi] section:
[midi]
# mpu401: Type of MPU-401 to emulate.
# Possible values: intelligent, uart, none.
# mididevice: Device that will receive the MIDI data from MPU-401.
# Possible values: default, win32, alsa, oss, coreaudio, coremidi, none.
# midiconfig: Special configuration options for the device driver. This is usually the id of the device you want to use. See README for details.

mpu401=intelligent
mididevice=default
midiconfig=

Set mididevice to coremidi and play with the mpu401 settings.

Hi Dominus. Thanks for replying 😀

I did change my dosbox settings a few days ago, which was necessary to get the MT-32 to work for the brief time it did. Here's what I ended up with:

mpu401=intelligent
mididevice=coremidi
midiconfig=0

Something else I've noticed: the Midisport Uno's midi out light does go active for just a second when I open dosbox. Still no luck when I run a game, though.

strange. If it wouldn't be working with ScummVM I'd say your Midisport is broken...

How are you running it? Just want to make sure you aren't doing something more complicated...
I've just hooked the MT32 up via another USB-Midi device and the line out of the MT32 to the Mac's line in and when I run a program that makes use of the MT32 (Dosbox or Exult) I start the LineIn app to hear the line-in on my speakers.

And did you restart the mac since it began behaving strangely? You never know, perhaps the Coremidi driver/framework got stuck or so... 😀

Hi again. My setup is exactly the same as yours, and I'm not using a usb hub or anything like that either. I did restart the computer, power down and cold boot, etc. too. I wonder if there's a way to update/reinstall the coremidi driver? I just keep coming back to the fact that test notes work the way they should, to reassure me the MT32 itself is all right, and that _some_ software on this machine can still send data to the Uno. Soooo weird 🙁

did you try

mixer /listmidi

inside DOSBox? Maybe your notes go to some other (probably virtual) MIDI device and end up in limbo somehow?

Okay, I accidentally fixed it. If anyone has this problem in the future (including me), here's what I did:

Disconnect the Midisport Uno
Delete everything in /Library/Audio/MIDI Drivers
Delete ~/Library/Preferences/com.apple.audio.AudioMIDISetup
Remove the Midisport Uno from Audio MIDI Setup devices
Reboot
Reconnect the Midisport Uno

I have no idea which of steps 2 through 4 was/were critical, or what caused the problem in the first place, but it's an easy fix at least. ScummVM and DOSBox are both making nice with the MT32 again. Thanks for everyone's suggestions! 😀

Nice that you figured it out