La música es un idioma entendido por todos los humanos.

English Translation

Music is a language understood by all humans.

It’s recipient, why isn’t it para?

Sometimes, por in Spanish works like by in English. For example,

▸ ¿puedes pasar por la tintorería?
could you call in at or drop by the drycleaner’s?
▸ agárralo por el mango
hold it by the handle

The way I see it, this is a passive construction, and the agent is indicated using “por”.