Sentence:  But whenever one of the motor-trucks lumbering by bore a big U.S. on its rear panel Troy pushed his light ambulance ahead and skimmed past, just for the joy of seeing the fresh young heads rising pyramid-wise about the sides of the lorry, hearing the snatches of familiar songs—"Hail, hail, the gang's all here!" and "We won't come back till it's over over here!"—and shouting back in reply to a <WSD>stentorian</WSD> "Hi, kid, beat it!", "Bet your life I will, old man!" 
Meaning of the word marked with <WSD> tags:  Loud, powerful, booming, suitable for giving speeches to large crowds.