I would start at the speaker. Pull one out and connect a known good speaker to it, can be almost any speaker to try temporarily even a house one. If still dead then you will need to work your way toward the head unit. From what I've seen a no/intermittent sound issue is sometimes a broken wire where the wire flexes between the door and chassis, a bad speaker or a loose connection. Speakers are pretty cheap to replace, Crutchfield breaks down the options well.