What was the first movie to use multitrack and playback audio?

Answer 1

In 1940, Walt Disney's Fantasia was the first film to be released in a multichannel format called Fantasound.

Other films used multi-track playback, but it was Apocalypse Now that spurred the direction of 5.1 stereo:

In the seventies, Coppola told Murch that he wanted the soundtrack to be in a new format. “He didn't specify exactly what that format was,” Murch said, “but he wanted the sounds of the conflict and the music to envelop and surround the audience in a very complex, powerful way.” As a result, Murch created the split-surround six-track format (what's now known as the 5.1 format, with .1 referring to the sixth, super-low-frequency-only, track). Because they used this format, which is now an industry standard, the audio didn't need to be remastered at all; it was simply transferred from the analog masters to the new Dolby SR/D digital format. The technology had finally caught up with the film.