During the 20th century, music was able to take off as a hobby for everyone. Just seven years before the start of the century, gramophones were introduced to the market. Then, twenty years into the 20th century, commercial radio was invented. The combination of these two inventions opened the musical door to people of low income because concerts and musical theatre, the most common places in which people listened to music, were mostly reserved for people of higher income.
Sound mixing and multitrack recording were created in the 1950s. Sound mixing allowed sound engineers to add more layers of vocals and other instruments to a recording. They could also create sounds that were not possible to make during a live performance. Multitrack recording allows different sources of sound to be recorded at different times. The separate recordings could then be brought together as a whole. This laid the foundation for many of the sounds so common in pop music and rock today.
During part of the 19th century, orchestras had a set of instruments that was rarely every changed. As the Romantic period took over, composers started to drift a little bit from that tradition, giving some flexibility to what instruments were chosen for a piece. By the 20th century, composers were basically hand-picking what instruments were in their piece. This shift from tradition allowed there to be new instruments to be used. Some pieces called for instruments like the electric guitar. The record turntable, used in hip-hop, was able to be used because of the wide-open array of instrumentation allowed in songs.
Composers started to break rules that were established in previous musical periods. New forms and styles were created, such as Bebop jazz and its use of altered and extended chords. Guitar amplifiers and sound systems made it possible to bands to showcase there music to everyone in the audience, even people who had the cheapest seats available.
Interestingly, neoclassicism arose around 1923 to 1950. Neoclassical composers wrote music in the style of the classical period with the intention of fighting back at "tasteless" Romantic music. In the second half of the century, similar styles of music were published but is referred to a postmodern music rather than neoclassical.
The 20th century brought in an era of change to music. New genres, styles, and instruments were introduced. Rules were broken. The foundations of contemporary music are rooted to the 20th century. The 20th century holds many wonders of music. Some of it affects history; some just the history of music alone.