Paul McCartney's collaboration with the surviving members of Nirvana wasn't as unlikely as it seemed on Wednesday night. Dave Grohl and Krist Novoselic joined the Beatle to close the 12-12-12 concert at Madison Square Garden. They sang 'Cut Me Some Slack,' which they wrote and recorded for Grohl's upcoming 'Sound City' documentary soundtrack, and the studio version is now available for sale on iTunes.

'Cut Me Some Slack' is included on 'Sound City -- Real to Reel,' a project that also includes Grohl collaborations with Stevie Nicks ('You Can't Fix This') and Rick Nielsen of Cheap Trick ('From Can to Can't). The website CutMeSomeSlack.net first revealed that McCartney is indeed on the studio version, something confirmed by the release of the track listing.

Nirvana recorded at Sound City, as did Neil Young, Fleetwood Mac, Tom Petty and dozens of other rock legends. Grohl talked about staging screenings of the movie that include performances from some of the artists on the soundtrack.

"After each premiere we could have a show with musicians," he tells Rolling Stone. "They'd not only play music from the film, but from their catalogs as well … We aren't far away from this. Pretty soon, everyone will hear what our big plan is. We've been planning it for a long f---ing time."

The 'Sound City' documentary is scheduled to premiere at the Sundance Film Festival in January, with the soundtrack to have an expected release date of March 12, 2013.