If you consider 1920 as roughly the beginning of the jazz style of American popular music, these early vocalists were strong influencers - mainly by changing the popular singing style into a more intimate, conversational delivery, helped by the invention of the microphone. Also this more relaxed delivery of lyrics fit with a new music style called swing which was based on 4/4 time.

The Jazz Age was a period in the 1920s and 1930s in which jazz music and dance styles became popular.  It’s often referred to as the Roaring Twenties and overlapped with the Prohibition Era (1920-1933) when the sale or consumption of alcohol was against the law.

Vaudeville, (a farce with music) is a form of light entertainment popular from the mid-1890s until the early 1930s. A show consisted of 10 to 15 individual unrelated acts, featuring magicians, acrobats, comedians, trained animals, jugglers, singers, and dancers. Today we’d call it a variety show.

Hollywood musicals were very popular from the B&W Fred Astaire-Ginger Rogers films of the ‘30s to the MGM color extravaganzas of the ‘40s and ‘50s. The “golden age” of Broadway musicals was in the 1940s-50s.