Speaking with Today's Martha Kearney, McCartney gave his thoughts on AI's "scary, but exciting" advancements before revealing that Peter Jackson, director of the 2022 Beatles documentary Get Back, was able to use AI tools to "extricate" John Lennon's vocals from a cassette demo. "He can separate [the instrument tracks] with AI. They tell the machine, 'That's a voice, this is a guitar, lose the guitar.'"
McCartney does not mention the name of the song, but the BBC has deduced that it may have its roots in a 1978 composition by John Lennon entitled "Now And Then." Lennon's widow Yoko Ono gave McCartney two cassettes containing the song in 1994; the following year, the surviving members of the band attempted to complete a studio version, but George Harrison's disdain for the recording quality of Lennon's demo led to the song's shelving.
Still, McCartney has since repeatedly expressed interest in completing "Now And Then." In a 2012 interview with BBC 4, McCartney said he would "finish it, one of these days" and repeated the sentiment in 2021 to The New Yorker.
Now, however, it seems that the demo's rough sonics will be less of an issue. "We were able to take John's voice and get it pure through this AI, McCartney says. "Then we can mix the record, as you would normally do. So it gives you some sort of leeway."
Listen to McCartney's interview here, with his comments on the new Beatles song beginning at 29:30.