MIDI software devices
DOSBOX currently does not provide emulation of general MIDI or the MT-32 Roland, instead passing the music data along to the MIDI device installed on your system. However, most modern PCs provide only very basic MIDI functionality, and therefore attempts to use general MIDI/MT-32 music under DOSBOX usually end up sounding rather poor.
Fortunately, at least for Windows users, there are a couple of free software projects that provide emulation of these MIDI devices, resulting in a great boost to 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, and these are not included with the download. Installation notes are provided within the distribution archive.
WinGroove has apparently been abandoned mid-development, but is still an excellent sounding general MIDI synthesizer. 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 of the process should be trouble free - but do not skip restarting Windows at the end.
Using MIDI software devices under DOSBOX
After installing either/both of these devices, you will need to make sure DOSBOX actually 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.