You can't do anything. Your neighbors can tell total lies to the police about you, and you could not sue them for anything. (It's still a crime, so don't do it yourself in return). If the police get annoyed enough, they could charge your neighbors with filing a false police report or merely put responding to them way down on the list of things to do.
You probably can't do anything. You could potentially seek a restraining order or injunction, but under the facts you provide, I doubt you would be successful. That could also be costly.
Calling the police isn't harassment for purposes of harassment restraining orders (it is allowed under the statute). Your only option would be to file a petition for injunction, which would be an expensive endeavor. If the police agree that your music isn't loud, I believe that they would have increasingly slower response times in order to discourage further reports. The only option left after these two would be to move.
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