Midi shows the 5 pin DIN plug, which at one time was also popular for stereo audio input and output connectors.

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MIDI composition takes advantage of the MIDI interface to allow musical data files to be shared among various electronic instruments by using a standard list of commands and parameters known as General MIDI (GM). Because the music is simply data and not actually recorded wave forms, it is therefore maintained in a small file format. Several computer programs allow manipulation of the data so that composing for an entire orchestra of synthesized instrument sounds is possible. The data can be reproduced by any electronic instrument that adheres to the GM standard.
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QUOTE
All MIDI compatible controllers, musical instruments, and MIDI-compatible software follow the same MIDI 1.0 specification, and thus interpret any given MIDI message the same way, and so can communicate with and understand each other. For example, if a note is played on a MIDI controller, it will sound at the right pitch on any MIDI instrument whose MIDI In connector is connected to the controller's MIDI Out connector.

When a musical performance is played on a MIDI instrument (or controller) it transmits MIDI channel messages from its MIDI Out connector. A typical MIDI channel message sequence corresponding to a key being struck and released on a keyboard is:

   1. The user presses the middle C key with a specific velocity (which is usually translated into the volume of the note but can also be used by the synthesiser to set characteristics of the timbre as well). ---> The instrument sends one Note-On message.
   2. The user changes the pressure applied on the key while holding it down - a technique called Aftertouch (can be repeated, optional). ---> The instrument sends one or more Aftertouch messages.
   3. The user releases the middle C key, again with the possibility of velocity of release controlling some parameters. ---> The instrument sends one Note-Off message.
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"Later - I think it goes into one of little circles with a pink, green or yellow ring around it. So cute. But this is the up-to-date method."

Those connectors are intended for 'normal' (wave format, not digitally encoded) audio. MIDI is a digital format and must be transformed into 'normal' audio.