A MIDI synthesizer in a computer is a device (whether or not built into a sound card) that generates midi events (note-on, note-off, select instrument, etc.) into an ordinary sound signal as can be reproduced through loudspeakers by an amplifier.
Music playback in MusiCAD always uses some kind of synthesizer (either internally in a sound card or via an external synthesizer connected using midi cables.
Sometimes several MIDI synthesizers are present in one computer, for example if both the motherboard contains a sound card chip and a sound card is also installed. Usually one of the two cards will be disabled. Another possibility is the presence of both a built-in synthesizer and an external MIDI synthesizer. Usually, a software wavetable synthesizer may also be installed.
To allow access to multiple MIDI synthesizers at the same time, Windows has a facility for this, the MIDI mapper, which controls MIDI input and output from programs such as MusiCAD and allows you to map part of the parts to the internal synthesizer. and send another part to a keyboard.
To know which MIDI facilities are present in the pomputer, go to the Windows control panel. There you choose [Sounds and audio devices] and choose the tab [Audio]
The pull-down menu under [MIDI Music Playback] should provide the same list as under midi dialog with the exception of the MIDI mapper that acts as a distribution station for the features shown, and a power-off position.