I imagine that the problem with Melkor's music was not that it was innovative (as you will recall that Melkor could only mimic and rearrange Eru's tunes, but could not create anything really
new) or that it was different, but that it was a deviation apart from Eru's plan. Melkor was over-eager and impatient, and ultimately unwilling to follow Eru's established timeline for the development of the theme. He had an agenda and a will which he set above Eru's. That is called rebellion, and it is why Melkor's foray into musical composition was wrong. It was a timing issue.
(Have I missed the point?
I don't to be redundant or preachy.)