In OM, most "score" objects can be played via a MIDI synthesizer. Objects are played :
With the space bar, or the p
/ s
keys from a patch editor.
Using the play / stop commands of the palette, or the space bar in the score editors
When thay are in a maquette, using the maquette playback commands
Playing an object in MIDI amounts to converting this object onto a sequence of MIDI events sent to a synthesizer.