I've heard upbeat songs in the context of sadness, for example at funerals in the Catholic church. But I don't think a song about someone being dragged into forced labor would be as upbeat as this one.
""slaves sing most when they are unhappy" sounds like quite a generalization, don't you think? Surely some might sing, others might not want to sing at all.
When I was in high school and did badly on a math test, I went into the darkened living room and listened to the saddest music I could find. Stravinsky, I think.