Yesterday was the last day I had to finish July's game. I had guitar lessons that very same day, so I showed the project to my teacher and asked questions about the choice of chords. He suggested using the I, IV, V, VI and VII grades of the minor scale, so when I got back home I just did that…
…just to notice that PICO-8 only supports 4 sound channels and I was trying to use 5 (one for drums, another one for bass, and three for the chords, since they were 3-note chords)… so there were sounds that were not being played. It was already late at night, so I opted to just use 5th chords: C5, F5, G5, Ab5 and Bb5. Next week in class I'll found how much of a crime I committed against Music.
But it worked!
Before going to bed, I tried the HTML5 export, and it seems to work as well. So I've uploaded it to itch.io, and you can play with it there.