Having done plenty of RAT tests where you spin it up using the aircraft system I can assure you I know the sound! Whilst people often go to pilots for crash incidents, their knowledge is flying not the engineering of aircraft. They will know if I press this button it will do this (they hope) they don't know how it works, that's of no interest to them in doing their job. Their whole job is about procedures and dealing with things when they go wrong. Pilots training has very little technical knowledge more so schematic drawings of the systems.I know!
Ok, I have no idea what sound they'd make.
He says he thinks it's engine failure, and gives some reasons. One of the reasons is that the RAT deploys. But he also says there are other things that could cause RAT deployment, so he needed more reasons as evidence that it was engine failure, not something else. If he noticed that the audio from the jet engines was far lower than it should have been, I'm surprised he didn't say so, that's all.
Most crashes are caused by human error whether pilot, engineer, loader, air traffic.