Experience suggests that one or more of the following common occurrences could have happened:
- the record head was out of alignment, dirty, or magnetized;
- that someone had tampered with the equipment (suspects brought in for interviews often try to sabotage the recording equipment while the detective is out of the room, such as by jamming a pencil into the microphone, so maybe the microphone had been somewhat damaged previously);
- that inferior quality tapes were used (just because you can get 100 tapes at the local big box store for $3 doesn't mean they should be used for what could end up being used as evidence);
- that the tape had not been changed recently (if their policy was to reuse tapes);
- there was an electrical power grounding problem (causing hum);
- the recorder had some other fault (bad tensioner maybe, but then the video would have been out of sync also); or
- the suspect was too far from the microphone to pick up clearly (doesn't sound like the case here though).