Can a tape recorded conversation be used in court?

My granddaughters mother is young and admits to smoking marajuana and states that she is addicted to it. She also states that her mother is aware of her doing this and doesn't do anyting about it. Yes I believe this as I have known the family for many many years. She won't go to school or try to get a job to take care of my granddaughter. My granddaughter is only 4 mos old. She doesn't belong with her mother. All of her bad habits and immaturity is all on a taped conversation. Can this be used against her in a court hearing as I am trying to gain custody of my granddaughter?