What is the definition of music​

an arrangement of sounds in patterns to be sung or played on instruments

Music is a form of art that uses sound organised in time. Music is also a form of entertainment that puts sounds together in a way that people like, find interesting or dance to. Most music includes people singing with their voices or playing musical instruments, such as the piano, guitar, drums or violin.

relief sculpture - images are set against a flat background.

a coin is a good example: the inscription, the date, and the figure are slightly raised above a flat surface. the ancient egyptians sometimes carved figures into a flat surface.  

pro: they take significantly less time to make and do not require as much tools or skills as free-standing

cons: not nearly as realistic, as it is not fully 3-d, and does not have the details that a free-standing one does.

freestanding sculptures (also called  carved in the round) are not attached to any  background.   they are 3-dimensional  and fully rendered.