I think Yoko (or was it John himself?) once said the gum made him remember the words to the songs. You can definitely hear him chewing in All You Need is Love, and if you listen closely, you can also hear the gum in I Am the Walrus. If anyone has a Bose player, that filters the sound, it's easier to hear.
"I feel like a princess" -- Sean Lennon, Irving Plaza, 4/13/07