In order to climb the MIDI learning curve, I started by exploring what Sparkfun has to offer for MIDI break-out boards. To get a proof-of concept working, I used the following components.

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MIDI SHIELD: Converts single wire TX/RX to the MIDI compatible electrical signal which operates over two wires in the 5-PIN round connector. In my test set-up, I'm transmitting MIDI through an SAO GPIO1 output with bit-banging [soft] serial from the Core64 Badge (could be any badge or serial output capable MCU board). The signal is converted to MIDI, goes to the "MIDI OUT" port through the MIDI cable, and then back into the "MIDI IN" port where the signal is converted back to a one-wire serial stream.

Credit: https://www.sparkfun.com/sparkfun-midi-shield.html

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WAV TRIGGER: Receives the one wire serial data command to turn notes on and off. The notes are played from a corresponding file on the SD Card. This is a product Sparkfun sells but it is designed and supported by www.RobertSonics.com - it's quite capable and even includes an onboard audio amplifier. Robert offers an impressive range of customizable boards like this.

Credit: https://www.sparkfun.com/wav-trigger.html

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The MIDI Library I am using on the Core64 (or Arduino UNO) is: 

https://github.com/FortySevenEffects/arduino_midi_library

This library has a lot of capability and seems to be the most common and best option for Arduino-based MIDI stuff.

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When it is all strung together, it looks like this:

The WAV Trigger board has an 1/8" audio output jack which is connected to the standalone portable speaker (blue). The Black "Uranus Z" is just a USB battery pack. And the Arduino UNO clone was what I started with to get the sample code running.

At this point, I can run the demo easily on an Raspberry Pi Pico (the MCU on the Core64) with the Philhower core (instead of using the Arduino/MBED core). I'm trying glue the demo code and my mainline Core64 code together now, so that each bit of core memory (and LED)  on the Core64 will trigger a MIDI note. This will enable the Core64 to behave like a MIDI Controller Grid:

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Here are some more resources for learning about MIDI:

https://learn.sparkfun.com/tutorials/midi-shield-hookup-guide

https://www.tigoe.com/pcomp/code/communication/midi/

https://codeandlife.com/2016/01/17/midi-to-usb-adapter-with-teensy-lc/

Talk MIDI with the Bus Pirate (on Talk Like a Pirate Day)

https://learn.sparkfun.com/tutorials/wav-trigger-hookup-guide-v11

https://www.polyphone.io/en/home

https://www.reddit.com/r/WeAreTheMusicMakers/comments/u3pblj/how_to_get_that_cheesy_1990s_videogame_midi_sound/