MIDI software devices
DOSBOX currently does not provide emulation of general MIDI devices or the MT-32 Roland, instead passing the MIDI data along to MIDI synthesizer installed on your system. However, most modern PCs provide only very basic MIDI functionality, and therefore attempting to use general MIDI/MT-32 music under DOSBOX usually results in poor sounding results.
Fortunately, there are a couple of free projects that provide software emulation of these MIDI devices, which provide a great boost to the music quality:
Munt is an open source project that aims to emulate the MT-32 hardware by way of a Windows XP WDM driver. Using it,however, requires a copy of the original ROM images from a Roland MT-32, which are not included in the software download. Installation notes are provided within the distribution archive.
WinGroove has apparently been abandoned mid-development, but still provides excellent sounding General Midi emulation. The latest available version, 0.A4 Beta-2, can be freely obtained from the site.
Installation note: After downloading wg0a4_e.exe, run it in order to extract the installation files into a temporary directory. Then, from within that directory, go to START -> RUN and type "WGINSTLR.EXE FULLSET.LST". The rest should be trouble free. You will need to restart your computer at the end, though.
Using MIDI software devices under DOSBOX
After installing either/both of these programs, you will need to make sure DOSBOX uses them. You can either edit the [MIDI] section in the .conf file (instruction are provided inside the file itself or in the README), or change the default MIDI device in Windows. To do the latter, Go to START -> SETTINGS -> CONTROL PANEL -> SOUND AND AUDIO DEVICES, select the "Audio" tab and choose the device you wish to use under the "Midi music playback" section.