Of several momentous developments in the film industry of the 1930s, two were of special significance for William Alwyn. The first was the growth of the documentary movement; the second was the invention and improvement of the sound track.

In the 1930s the documentary idea, as defined by the “father of documentary” John Grierson and his followers, was specific: a documentary was rooted in realism and a sense of social purpose. The movement was centred on the Film Unit of the General Post Office, a valuable school for a small coterie of university-educated, socialist-minded directors like Basil Wright, Arthur Elton, Stuart Legg, Edgar Anstey, John Taylor, and Harry Watt. They joined the Unit abysmally inexperienced and were paid about the same as a train driver. Some left to find sponsorship from industrial companies or associations - airlines, shipyards, travel groups, or in coal, oil, tea, gas, or chemicals, and it was from this offshoot that Alwyn found commissions. By the time he joined the movement in 1936, the Grierson school, both inside and outside the GPO, had produced its most interesting pre-war work, with titles such as Coal Face (1935), Housing Problems (1935), Night Mail (1936), and Enough to Eat? (1936). Significantly, they were films made possible only because documentary could at last afford a sound track.

Documentary sound was an infant compared with the entertainment film. At the GPO, Grierson had no access to sound-recording equipment until January 1934; until then, sound was added by distributors, over whom the producers had little control. Once sound arrived, important composers began to join the movement. As the Brazilian director Alberto Cavalcanti, at this time working for Grierson, remarked:

Cavalcanti could also have mentioned that they had Marius Francois Gaillard and Brian Easdale. Soon the other units outside the GPO also acquired sound equipment: GB-Instructional was joined by Arthur Benjamin and Jack Beaver; Matyas Sieber could be found at Strand; and Paul Rotha employed Leigh, Francis Chagrin, Hubert Davis, Clarence Raybould, and others.