Do You, or Have You Ever, Lived in the United States? Then You Are a Filthy Criminal.

[Husar]

sharing music online. (which you could be doing by simply playing your stereo too loudly at a party) and nobody has lost anything material. illegal.

Unless it's human beatbox music they usually bought some instruments, booked or bought a studio etc. to record that music, why is it wrong if they want something in return for those investments? So yeah, they do lose something material because they invested money to make that music in the first place.
And those who just make music because they love doing it and want to make people happy won't ask for money anyway and thus sharing their music is not illegal.