In order to answer your question, you need to carefully read your presumed written lease that you have with your landlord in that it controls the obligations owed to you by the landlord and vice versa in the absence of conflicting state law. Look to see if there is any reference about unreasonable noise.If there is no provision in the lease as to noise restrictions and your piano playing does not violate any local ordinance then your have done nothing wrong. I would write property management a letter in response. It seems that the only reason you are being targeted is due to the one neighbor who sleeps during the day that does not appreciate your music.