"You can lead a horse to water, but you can't make him drink."
If kids don't care, no amount of you simply saying "CARE ABOUT THIS" will cause them to care. Your job is to find out how to cause them to care or to make them care. If grades don't matter, and parent contact doesn't matter, and other classes don't matter, then figure out why they're at school.
Maybe they're in athletics or academic UIL. Talk to their coach and say "this kid will fail if they don't do their job in my class."
Maybe they care about the social aspect. "You can't go to (insert trip here) if you aren't passing." Then if they don't do their work, fail them.
Band shouldn't be an automatic "A" for every kid. If they aren't doing their work, fail them. Call the parents ahead of time and do your due diligence, but hold your kids accountable.
Finally, make sure these kids are in another elective next year unless they "see the light." They'll poison your ensemble and the HS band for the next five-eight years. An entitlement mentality is almost impossible to break.