The music for most of the Bond films through Diamonds Are Forever had been scored by John Barry. Due to his working on a musical, and having fallen out with Bond producer Harry Saltzman over the last title song, Barry was unavailable to score Live and Let Die.

Producers Saltzman and Albert Broccoli approached Paul McCartney to write the theme song and McCartney in turn asked Martin to record it for him. Impressed with the orchestration for the finished track, Saltzman and Broccoli considered Martin for the film’s score.

Martin worked closely with director Guy Hamilton who described what the music should convey in each scene as it unfolds. Only very minor changes to the finished score were asked for. Martin felt that this was as much for Hamilton's accurate briefing.

The orchestra was conducted by Martin and recorded at AIR studios. The soundtrack was also released in quadrophonic.

Having recorded McCartney's performance, Martin was taken aback when Saltzman asked him who he thought should sing the film's title song, suggesting to him Thelma Houston. Saltzman had envisaged a female soul singer. Martin said that it should be McCartney. He nonetheless scored a soul arrangement to accompany singer B. J. Arnau for a nightclub sequence in the film.

Live and Let Die was the first time that a rock music arrangement was used to open a Bond film. It was also the first time that McCartney and Martin had worked together since Abbey Road in 1969. McCartney had been considered as title song composer for the previous Bond film, Diamonds Are Forever. The song was nominated for an Academy Award but lost to "The Way We Were".

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Wikipedia