Tape recordings are regarded as 'documents' in English law and are generally accepted in courts of law. (However it would not be unusual for one side to argue that the tapes should be regarded as inadmissible by the court. Under such circumstances the judge might decide to listen to the tapes before making a decision as to their admissibility).
Since CCTV recordings (often with sound) are frequently used in court cases, it would be illogical to universally exclude sound-only recordings from being admitted as evidence.
There have been two murder cases, within the past year, where the police used covert sound recordings to bring a case before the courts. In one case a man was recorded talking to himself in his car, apparently confessing to murder. In the other, the defendant had been recorded in his house, talking to his cat about how he'd murdered his wife. I can recall that one case resulted in an acquittal, with the other resulting in a conviction, but I can't remember which was which.
Chris