No cvr recording, but obviously working on prior flight. Both engines showing no signs of pre impact failure.
Very odd.
It doesn't surprise me in the slightest. I flew 737's for many years and after reading an article on testing the CVR for one's own voice transmission, the very first 737 CVR I checked by calling "test test test" into the CVR microphone and expecting after two seconds to hear my own voice, revealed no voice. I snagged it in the maintenance record and a defect was found by the technicians and fixed. In fact this happened on numerous occasions on other 737's I flew yet the test needle indicated the CVR was serviceable.
Then I flew for a European airline that had recently bought new 737-500's
I flew one which only had about 40 hours since new. I decided to plug my headset into the CVR jack in the cockpit and check my own voice for clarity.
Guess what? No voice re-transmission. Over the next few months checked the CVRs on other new 737-500's and a couple of 737-400's of that airline and on some discovered identical defects on their CVR voice test. Others were perfectly clear speech re-transmissions and some where the speech was so muffled as to be useless. After those experiences I made it my own SOP to check the correct operation of the CVR before every flight. Over many years I found many CVR failed to record one's voice.
Just because the Boeing 737 (older types anyway) passed the FCOM Supplementary Procedures test of pushing the CVR test switch and receiving a tone through a headset plugged into the headset jack as well as noting the Monitor indicator in the Green band, it doesn't necessarily ensure your voice is recorded or legible.