You would need to prove that you had never heard it. This is non-trivial.
Most recent high-profile music copyright cases do not center on precise reproductions, but on music that "contains elements of" another piece of music.
Anyone can find themselves forced to defend their innocence after being hit with DMCA notices, but content creators are especially vulnerable because they depend on their works being publicly available. They have a lot more at stake (including the ability to eat and pay rent) if they are hit with accusations.
They are the ones who'll be hit by poorly implemented algorithms (like Google's notoriously terrible content ID or the many even worse imitations created by companies without the talent/money/data google has at their disposal) or can be targeted specifically because their creative works are perceived as a threat to some company.
The DMCA is often abused to silence criticism, to hide unpleasant information about a company or individual from the public, or to attack competitors. It costs very little to fire off accusations that have real-world consequences and as far as I know, no major company has ever been held accountable for doing so inappropriately, even when it's been brought to the attention of courts.
I could reasonably prove to a court I've never been to Jupiter, or that I've never been President of the USA, etc. I can definitely think of situations where someone could likewise reasonably prove to a court they've never heard a song (e.g. they're deaf) or their "AI" had never heard a song (e.g. showing the training data).