By now it's a known fact that the majority of The Beatles' albums were originally mixed and prepared for mono output, not stereo. Stereo was a relative novelty until the late 1960s, and people like recording engineer Geoff Emerick (who worked closely with the Fab Four on such landmark albums as Revolver and Sgt. Pepper) have said that The Beatles themselves were intimately involved in the process of mixing the mono albums, while they were content to leave the stereo mixes to the "white lab coats" at EMI studios.
The end result is that many of the mono versions of well-known Beatles songs -- previously unavailable outside of the original vinyl records, prior to the 2009 remaster project -- contain hidden sonic gems that do not exist in the stereo versions.
Here are five "Easter Eggs" in the mono versions of Beatles songs that you've probably never noticed before:
Sing along with me on the third verse: "Sky of bluuuue, and sea of green ... in our yellooow ... submarine!" Except, wait, rewind that a bit. Yep, Ringo flubbed the last word, and accidentally sang "slub-marine" instead of "submarine." Nice one, Ringo.
Good Morning, Good Morning/Sgt. Pepper Reprise
Producer George Martin has spoken many times about the bit of "good luck" he had in discovering that the final chicken squawk at the end of "Good Morning, Good Morning" blended perfectly with the opening bent guitar note that kicks off the "Sgt. Pepper" reprise. Listen to it in the stereo version and it's a flawless transition between the tracks. Listen to it in the mono version, and whoops, that's a surprisingly sloppy edit that probably shouldn't have made the final cut.
Norwegian Wood (This Bird Has Flown)
George Harrison famously popularized the sitar with the modern rock music audience by including it on this song. But listen closely to the mono version (you may need to turn it up just a bit), and after the lyric "she asked me stay and she told me to sit anywhere," you'll hear George cough. Someone in the mixing labs must have caught this during the stereo mixing, because you won't find it there.
There's not one isolated instance of "Easter Egg" discovery here, it's the whole track that deserves mention. The mono version of the song uses a different vocal take for John's lead than the stereo version does, and the mono version is downright gritty. Listen especially to the second instance of the chorus: when John sings "Help me if you can," he practically barks the word, and he actually sounds like he might be in need of some help.
Chalk it up to better mixing decisions, or chalk it up to simply hearing a familiar song in a new way, but the mono version of this song far more powerfully captures Paul McCartney's intention to record one of the heaviest, nastiest, grittiest songs ever. The bass is louder, the background vocals shriek and swoop with a presence that will give you goosebumps, and the lead vocal is so up-front-and-center you'll feel like you're sitting nose-to-nose with Macca himself. Bonus: Ringo's screamed "I've got blisters on my fingers!" is not present on the mono mix, which, depending on your preference, might make the track better or worse.