No need to annotate the chords. If you start from a midi file, you just encode the piece as a binary matrix (time range x frequency range). It gets trickier if

No need to annotate the chords. If you start from a midi file, you just encode the piece as a binary matrix (time range x frequency range). It gets trickier if different instruments are not separated because you would want to specify different instruments as separate channels (so your matrix becomes time range x frequency range x number of channels), but even this could be automated with ML (e.g. using image segmentation methods). The most time-consuming part is finding enough training compositions with labels (such as composer, genre, mood, style, etc), and data cleanup.