[josephlord]

Usually correct I imagine but what about the slow de-pressurisation scenarios where the pilots have passed out and the plane flies until it runs out of fuel?

In this case (whatever the cause) it would definitely be useful to have the recording for the time the plane went off the planned route but that will probably have been overwritten.

[jryle80]

That's a good point. However I believe low pressurization event is captured by the data recorder. That information, coupled with other events, and the fact that the voice recorder goes silent for 2 hours, would give investigators sufficient clue.