Who sings it?
That would be Rodrigo Amarante, a Brazilian singer-songwriter who's been on the music scene since the late '90s. He got his start in the Rio de Janeiro-based band Los Hermanos, then joined Little Joy, which paired him with the Strokes' Fabrizio Moretti. He went solo in 2014 with the album Cavalo. You can hear a few tracks off that record via NPR's Tiny Desk Concert series. (Fun fact: the guitar Amarante plays there has a name, and it's Butter.)
Narcos producer José Padilha was familiar with Amarante's music because his cousin made a documentary about Amarante's band. But Amarante had also worked with Wagner Moura, who played Escobar on the show (RIP), on a Brazilian production of Hamlet. With recommendations from both Moura and Padilha, Amarante was a natural pick for the title music.