That's a rare problem, I'd say. This error message is shown when the 64-bit MIDI driver DLL (mt32emu.dll) cannot be copied to the Windows System32 directory for any reason. See if your user has permissions to write there (perhaps, some AV stuff blocking access?), or maybe an older version of the driver already exists in Windows/System32 and the update function fails to register a pending deletion on reboot.

If it's supposed to be at the top of the system32 folder, and not deeper in, then it doesn't look like I have it there. Is there a way to download that file individually and manually copy it there? I don't think I have any issues with permissions, although I'm not quite sure what you mean by AV stuff. Thanks for the reply!

Is there a way to download that file individually and manually copy it there?

Sure, you can download a dedicated package with the driver only from here. Moreover, the driver DLLs must be installed with the package into Program Files/munt//mt32emu_win32drv where you should be able to copy it from. Note, the 64-bit DLL is to be renamed mt32emu_x64.dll -> mt32emu.dll

Hope this helps.

I copied the mt32emu.dll to the system32 folder, and the mt32emu_x64.dll to the syswow64 folder, renaming it to mt32emu.dll like you said, and then restarted my computer for good measure. But dosbox still doesn't find it with mixer /listmidi. Did I do this right? If so, any other ideas?

I copied the mt32emu.dll to the system32 folder, and the mt32emu_x64.dll to the syswow64 folder, renaming it to mt32emu.dll like you said, and then restarted my computer for good measure.

Sadly, this is not what I said. Thanks to Microsoft, System32 contains 64-bit files and syswow64 in contrast - 32-bit files. So you've copied driver to wrong place 😀

Still, the setup tool is intended to do that for you automatically, no idea what's wrong. You can also try running "infinstall_x64.exe install" from the driver directory. As this alternative tool uses a different installer technology, it might do a better job, who knows.

Infinstall_x64.exe install didn't work, it gave me this error: Cannot install MT32Emu MIDI Driver; Error: e0000247. Luckily, I tried switching the .dll files around to their correct (yet agreeably aggravating) folders, and that actually worked! Dosbox shows "MT-32 Synth Emulator" as an option when using mixer /listmidi. Problem solved! Although, I wish I knew why my computer just refused to install those files correctly. Oh well 😜 Thanks for all your help, you've been wonderful!

Infinstall_x64.exe install didn't work, it gave me this error: Cannot install MT32Emu MIDI Driver; Error: e0000247.

Argh, right... It needs a signed driver first time you install it, so the correct spell was "infinstall_x64.exe install -sign" actually. Sorry about that (although that's all written down in the readme 😉 ).