If I'm going to go back in time to see/hear a musical performance, I'd like to go waaaay back. I would like to back to the later part of the 1700's and see the first performance of Mozart's Eine kleine Nachtmusik. I'm not a really a huge fan of classical music but this serenade is by far my favorite classical piece.
My second choice would be to see The Beatles in the studio recording either the Sgt. Pepper's Lonely Hearts Club Band or The White albums. I think that it would have been really interesting to see them recording. Plus it would irritate Gary to no end.