Because all of the Beatles' master recordings were originally recorded on 4-track and 2-track equipment, multiple instruments shared the same tapes. In order for the music to work with Rock Band, Giles Martin, son of the George Martin who oversaw the original production of most of the Beatles' music, had to spend months using digital filters to create separate tracks for each instrument.
Contributed by Pogue-Mahone
Paul McCartney gave himself the role of "Executive Tweaker" when working on the game.

"McCartney told me that his role in the project was 'executive tweaker,' but the better title might be watchdog." -Giles Martin, Music Producer for The Beatles: Rock Band
Contributed by Pogue-Mahone
Every individual digital track made from the Beatles master recordings that were sent to Harmonix to make The Beatles: Rock Band were low-fidelity copies imprinted with static. This was a security measure to assure that The Beatles' music wouldn't be sampled by other artists in the event that the recordings were leaked.

These low-quality copies were used throughout development, and replaced with high-quality versions for production.
Contributed by Pogue-Mahone
Before starting a song in any of the Studio Stages, the voices of the members of The Beatles practicing and talking to each other can be heard. These are actual recordings of the band members that were never released to the public before.
Contributed by Pogue-Mahone
Plans for a Beatles themed Rock Band game happened before the first Rock Band game was ever released thanks to a meeting between Dhani Harrison (George Harrison's son) and MTV Executive Van Toffler at a Christmas Party in 2006.
Contributed by Pogue-Mahone