You are almost right. Your only mistake IMO is in confounding Music with "Music composition":

To use your expression, Music (and not sound) is indeed, "perhaps counter-intuitively, harder than image."

You also correctly stated the right reason why: "difference in complexity between the two types of compositions"

However, 40 Years of Electronic Music and Hip Hop music have utterly demonstrated how forgiving we can be of sounds like Cheesy synths and low-res samples loops, if they are culturally "organized".

This is good news for AI!

At least in the short term, the real creative potential of AI in music is in manipulating "Musical Sounds", not in generating "Musical Composition".

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PS : I've recently made a short list of creative musical AI tools here https://rentry.co/Music-Creation-AI-Tool .