Isn't that claim contested? I think there are schedules which list audio recordings being taped? Someone is going to have to put the mona lisa button to use. Jam, you're the best person for finding stuff on here... 
It seems it is contested by Mike and in GDS's wireless log and echoed by Andrew Hunter, although like everything else, it's simply defence allegations without supporting evidence.
The defence appear to be arguing that they didn't know about the recordings until Bonnett made a statement in 2001 indicating that calls were routinely recorded, the defence then goes on to complain that the tapes should be disclosed, alleging that they are being withheld in order to keep Jeremy in prison.
The police (as mentioned above), indicate in the Dickinson report that the recordings were destroyed/recorded over as they were not requested within one month.
Mike has then produced this document below, citing it as proof that the recordings were still around a month and six days after the murders, although the reality is that it simply indicates that the police first sought the tapes a month and six days after the murders, which fits in with the explanation that they were destroyed/recorded over as they were not requested within one month.

That's about all I can dig up at the moment.
