"I have seen no mention of comparing CVR sounds to those on the CVR for JAL.
I assume that flight was airborne after the aft bulkhead failure for a
sufficiently long time that the tape covering event onset was recorded over."
About 32 minutes from failure to crash. Nevertheless it was reported that the
bulkhead rupture was audible on the Collins CVR (which I suppose from the era
was on a 30 minute loop). Also reported audible: a cabin decompression alarm
and the oxygen mask PA announcement.
In theory, data that has been overwritten can be recovered with as little
as 15% degradation. In practice several incidents suggest that the recovery
of data that has been overwritten can be problematic - incidents such as
runway excursions on landing after which the CVR was allowed to continue
taping.
For comparison, the US Govt protocol for "shredding" classified computer
files calls for overwriting each byte four times in a specified pattern.