Which 1927 film is considered the first feature-length 'talkie'?

Answer

It had synchronized singing and some speech.

Context

The Jazz Singer, directed by Alan Crosland for Warner Bros. and starring Al Jolson, used Vitaphone, a sound-on-disc system in which the audio was played from a separate record synchronised with the projector. It was the system's second feature, after Don Juan (1926), which carried music and effects but no speech.

The label first talkie is therefore somewhat misleading. Britannica states that only the musical numbers and some conversations were recorded, about a quarter of the running time, and that other films had already used synchronised sound. Lights of New York (1928) was the first Vitaphone feature with all of its dialogue recorded.

The film is remembered for Jolson's line You ain't heard nothin' yet, which came to symbolise the arrival of sound pictures. Its success moved the small studio into the ranks of the majors and earned Warner Bros. an honorary Academy Award. Eddie Cantor and George Jessel both turned down the lead role before Jolson took it.

Sound-on-disc proved fragile, since discs could drift out of sync with the picture, and the industry later standardised on sound printed directly on the film strip. Jolson's blackface performance in the film is still studied for what it shows about stereotypes and assimilation in American entertainment.