This may not be too weird. My mother has 2 dogs, and each evening, one brings a squeaky toy for my mom to squeeze, and they sing along. It only lasts for a minute, but they think it's great fun. I would assume that:
1. The crying hurts your dogs ears (which I doubt as you think he may be singing)
2. He's alerting you that one of the kids is crying
3. He wants to join in what he perceives to be "the fun" - this would be my guess.
I don't have a clue as to how you can change it. Maybe you should just howl along (only kidding)