Sometimes a CVR has to be taken to the manufacturer to get the data as in this case.
"While attempting to download the information from the CVR, it was found that the recorded
data had been erased. The CVR was taken to the manufacturer’s facility in Seattle and the data
was successfully recovered. The CVR showed that 31 seconds before the end of the recording
the CVR was powered down; the time recorded on the CVR from the aircraft UTC clock was
21:33:39. The CVR was then powered up again at 21:49:36, erased twice and then powered down
six seconds after the second erase command was completed."
http://www.tsb.gc.ca/eng/rapports-re...8/a04o0188.pdf
So for anyone who thinks erasing a CVR might get rid of the evidence, it is still retrievable.