Harmonicas were heard on a handful of recordings in the early 1900s, generally labeled as a "Mouth Organ". The first jazz or traditional music recordings of harmonicas were made in the U.S. in the mid-1920s. These recordings are 'race-records', intended for the black market of the southern states with solo recordings by DeFord Bailey, duo recordings with a guitarist Hammie Nixon, Walter Horton, Sonny Terry, as well as hillbilly styles recorded for white audiences, by Frank Hutchison, Gwen Foster and several other musicians. There are also recordings featuring the harmonica in jug bands, of which the Memphis Jug Band is the most famous. But the harmonica still represented a toy instrument in those years and was associated with the poor. It is also during those years that musicians started experimenting with new techniques such as tongue-blocking, hand effects and the most important innovation of all, the 2nd position, or cross-harp.
The harmonica's versatility brought it to the attention of classical music during the 1930s. American Larry Adler was one of the first harmonica players to perform major works written for the instrument by the composers Ralph Vaughan Williams, Malcolm Arnold, Darius Milhaud and Arthur Benjamin.
The harmonica made its way with the blues and black migrants to the north, mainly to Chicago but also Detroit, St. Louis, and New York. The music played by African Americans increasingly began to use electric amplification for the guitar, harp, double bass, and a crude PA system for the vocals. Alec Rice Miller, better known as Sonny Boy Williamson II, was one of the important harmonicists of this era. Using a full blues band, he became a popular act in the South, with his daily broadcasts on the King Biscuit Time show originating live from Helena, Arkansas. He also helped to popularize the cross-harp technique, which became an important blues harmonica technique.
A young harmonicist named Marion "Little Walter" Jacobs revolutionized the instrument by playing the harmonica with a microphone (typically a "Bullet" microphone marketed for use by radio taxi dispatchers cupped in his hands with the harmonica into a tube amplifier, giving it a "punchy" mid-range sound that can be heard above radio static, or an electric guitar). He cupped his hands around the instrument, tightening the air around the harp, giving it a powerful, distorted sound, somewhat reminiscent of a saxophone.