Originally all words had one pronunciation each (一個蘿蔔一個坑 each radish has its hole, couldn’t resist that), but as the language evolved, characters took on different pronunciations for the sake of clarity.
Or perhaps we should say, since the written language follows the spoken language, as language evolved, people started pronouncing words a bit different for the sake of clarity.
All the various pronunciations of one character have their logic, based on the early pronunciation of the word. 樂 was probably pronounced something along the lines of *nguk or even *ngruk, but eventually, for music it became yuè, because Mandarin eventually lost initial ㄫ/ng sound, and the final k entering tone, so what was left was yuè. But “happy” developed in another direction, lè. Then you get enjoy, yào, and in some contexts it was even borrowed for 療 liáo. Without boring you (and me) with all the phonological details, these all developed from the original sound.
But isn’t that wonderful? Music and Happy were originally the same word. We would do well to keep that in mind; rather, parents who force their kids to study piano or violin should keep that in mind.
Here’s the turtle bone form of 樂, some sort of stringed musical instrument.