Despite the mysterious aura, “Nobody Knows I’m Here” wastes little time establishing Memo’s backstory: Grainy video recounts the melodic voice of his childhood, and how his father struggled to make a buck off it after the business determined that the boy wasn’t photogenic enough for primetime. But when a young stud is chosen to lip-sync to recordings of Memo’s voice, the child lashes out in a violent act that destroys both of their lives.