Harry had a stupid idea, it would work, but it was stupid.(like most of his plans) He remembered reading (what a shock!) about a spell (ventriloquist) that could throw one’s voice. He figures it would be a great distraction if he ever needed one. This is what leads to him using it when he’s ‘dead’ in Hagrid's arms. He sings to try to distract and to warn Ron and Hermione he was up