Motives aside isn't a valid reduction of their action, the motives you have when you take an action are very much a part of whether that action is illegal. This is why both murder and manslaughter charges exist as separate crimes in most legal jurisdictions.

I don't know if this would be strictly illegal - it'll depend on what, precisely, police officers are required to do w.r.t. civilian oversight and it might require specific legislation to resolve. However, the intent of the police officers is to prevent people from filming, if they are attempting to accomplish this through playing music because more direct actions have been restricted then a court is probably not going to look kindly on their actions. As a parallel if you are barred from dumping hazardous materials on state park land and instead dump that on an interstate that passes through state park land a court will likely find you in violation.