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It would be invalid in the sense that it could not be used as evidence in any court of law in Maryland. It is not a criminal offense if you both were in Arkansas at the time of recording. It would be a criminal offense under both Maryland and federal law if one of you were in Maryland and one in Arkansas at the time of recording. But if the employer suggests using this for some reason of your employment, they can...this is because an employer isn't a court or bound by criminal law in what evidence is accepts for an employment action.