Final answer:
Popular music in the 20th Century greatly influenced and reflected societal changes, with mass media like the radio elevating its importance by broadcasting to a wide audience, marking a shift from the live and sheet music-dominated earlier periods.
Step-by-step explanation:
The importance of popular music to 20th Century audiences was significant as it not only served as entertainment but also reflected and influenced society's evolving norms and values. Unlike earlier periods, where music was primarily live and sheet music sales were common, the 20th century experienced the advent of mass media, which allowed music to be broadcast to wide audiences.
Technologies like the radio became the main source of entertainment and news, and later, television and recorded music formats expanded the reach and influence of popular music even further. Jazz music in the 1920s, for example, paved the way for modern genres and bore witness to America's growing secularism and the challenges between traditionalism and modernism.
Music in the 20th century was also a vehicle for consumerism and cultural exchange, as seen in its contribution to the war effort, influencing feelings of patriotism through popular songs and rallying the public.