Just a quick update this time: A long while ago I made a post about using Adafruit Trinket without Arduino and later converted that into a TrinketMIDI Github repository for making a MIDI device with ATtiny:

https://github.com/jokkebk/TrinketMIDI

Now thanks to a contribution by Gerhard Zintel, there is now also a MIDI volume device sample code in the repo. If you want to make a MIDI volume controller, it should be pretty easy with the code as well. Enjoy!