MIDI is not audio, and by itself makes no sound. MIDI is control information only. It is like the piano roll for a player piano; it provides control information for what note to play when, for how long, at what volume, and with what sound (instrument). For example, when you strike a key on a MIDI keyboard, it sends a message to a MIDI instrument to play that particular note at that particular velocity using the selected sound (instrument). This could be its internal tone generator (like a synthesizer or sampler), another external MIDI instrument, or an instrument plug-in within Pro Tools. In order to play and hear a MIDI recording, you must have a MIDI instrument. Audio from an external MIDI instrument can be sent to an external mixer or monitored through your Pro Tools audio interface (using either an Instrument track or Auxiliary track).If you are using an external MIDI instrument, it must be connected to MIDI ports that are recognized by your computer. These ports can be on a Pro Tools interface that has MIDI ports (such as an Mbox Pro) or some other MIDI interface (such as an M-Audio MIDISPORT).To actually hear an external MIDI instrument, you need to connect its audio outputs to a mixing console or connect it to one of the audio inputs of your Pro Tools audio interface.Just as each Pro Tools system has unique hardware features, each MIDI device has its own features (and limitations) as to the number of voices and instruments it can play at one time. See the device’s documentation for information on its capabilities.