To -J. Sounds like these bozos are plain rude cretins. I think it is inexcusible behavior to play over a singer. Or, maybe they are just inexperienced and don't ralize they're out shouting you so to speak. If the latter is the case, then perhaps a diplomatic word and a gentle smile might be the solution. If the former is the case, find some new friends to sing with. Some folks spend whole lifetimes learning how to accompany singers. It isn't always easy.