Although The Beatles had finished the music for the song Sgt Pepper's Lonely Hearts Club Band on 3 March 1967, it still wasn't complete. During this session they added sound effects to give the impression of Sgt Pepper's band putting on a concert.
The sound of the band warming up came from the 10 February orchestral overdub for A Day In The Life - the session musicians had been recorded before the overdub began in case the sounds proved useful at a later date.
Sgt Pepper's Lonely Hearts Club Band also required the sound of an audience taking their seats, applauding and laughing. These came from EMI's archive library of sound effects - the tapes in question were Volume 28: Audience Applause and Atmosphere, Royal Albert Hall and Queen Elizabeth Hall, and Volume 6: Applause and Laughter.
Towards the end of the session, which ran until 12.30am, two mono and eight stereo mixes of the song were made. These were numbered 2-3 and 1-8 respectively, and the last of each was used on the album.