The final info on the CVR was the snap of the seatbelt being unbuckled as the PIC prepared to leave the cockpit. Approx 5 minutes later, the F/O replied in a normal voice to Jakarta ATC (source: ATC tapes).
When the PIC returned to the cockpit, the most likely scenario involved his advising the F/O that he was needed in the cabin regarding a leaking tap etc.
Pulling the CVR cb was obviously not noticed by the F/O. Pulling the DFDR cb, however, causes a MASTER CAUTION light to illuminate and that was why the PIC wanted the F/O out of the cockpit.
All suggestion and scenario, but the most probable.