If you want an exception to copyright, you're going to want to start looking at a section 107 (of the copyright act) exception: https://www.copyright.gov/title17/92chap1.html#107

The reason someone walking down the street and humming a song is not a violation is because it very clearly meets all of the tests in section 107.

The biggest problem with feeding stuff through a black box like an LLM is it isn't easy for a human to determine how close the result is to the original. An LLM could act like a Xerox machine, and it won't tell you.