Dan is right, it is really complicated. With music copyright, the fair use clause is relevant. The difficulty lies in proving music original, as there are only so many combinations of notes.

There are lots of composers who steal other composers stuff, particularly ones out of copyright from the past.

A composer can be really unethical, really tricky and get away with it.

Also there are a lot of \coincidences'in music. Like many fields, developments are timely and often happen in different places by people unaware of each other at the same time. Gives a new slant on the word co-incidence.

Largely, really good musicians can tell derivative works from 20 paces.

And generally a rule of thumb is that people don't like new music, because it is unfamiliar.

With artwork, I think you can be as influenced as you like by other people's work, but if you reproduce someone else''s work then if it'is still in copyright (which lapses with time say around 50 years) you are in breach.

However policing this stuff is ridiculous, and real artists and composers don't usually get too concerned, because they are usually so out of step with the average works, that they are not generally popular at first.