Enjolras would belt out Imagine Dragons songs in the shower. And Come On, Eileen. But then he'd totally deny it when R hears him singing it one morning when he woke up on E's couch after getting too drunk to go back to his own place. E only sang it because Courfeyrac and everyone are always singing it all the time and he can't get it out of his head.
And then of course R starts singing Hallelujah by Jeff Buckley in the shower and E hears him one other morning and and realizes that wow, R was a beautiful voice and so he starts paying more attention when when R is humming in class or at work or while he's cooking. And he's started hearing R when he mutters E's name in his sleep. But he'd never tell anyone.
Because R doesn't even know he's doing it. It's second nature, habit even, like writing "apollo" on his wrist everday or putting on his green hoodie every morning. It's routine. Comfortable. Like breathing.