At the end of “Poison Root,” the opening track on Alex Giannascoli’s new album, Rocket, the 23-year-old artist repeats the phrase “Now, I know everything” again and again, his voice seething over a clatter of banjo, violin, and acoustic guitar sounds. It’s difficult to ascertain the exact tone: does he really think he knows everything? Or are these incantations a form of self-assurance, covering up insecurity? The tension between ambition and self-doubt in this closing refrain is typical of Rocket’s fourteen tracks. Over musical backdrops that effortlessly jump from sound collage to country pop to dreamy folk music, the cast...