This is a jukugo word, which usually means on'yomi readings from the kanji. You haven't learned this reading for 楽 yet, so here's a mnemonic to help you:
If you play your music too loud, people will come and gawk (がく) at you. But if you try to turn it off and say, "OH WHAT I DON'T LIKE MUSIC ANYWAY!" they'll gawk at you even more. Learn proper music etiquette and people won't gawk at you so much, okay?
I’ll listen to music after I study.
Do you think it isn't good to work while listening to music?
I wonder if another type of music will be created. Do you think mankind already made every possible genre?