To all the deniers out there who express doubts as to what may or may not have been picked up by the CVR, it is possible to build a small kit capable of detecting a human heart beat at 25 metres using just easily available consumer grade parts and a couple of cheap electret microphones the size of a pea.
If a cheap consumer microphone element costing less than a pound and mounted on an open circuit board can perform this well, how much better will a commercial grade microphone insert work?
Using multiple speakers placed where the original recording microphones were located, a soundscape can be recreated allowing precise estimates to be made of the placement of all sounds recorded.
The only limitation is the dynamic range available in the recording, which limits the amount of digital filtering and processing that can be reasonably applied.
It is perfectly possible to identify and place a soft key click to a given area in the soundscape, especially if background noise has been heavily filtered. Breathing sounds are a cinch. Given sufficient gain, the brush of fingertips against a control surface can be identified and located to a given point in space.
Any audiophile with a high end system can easily demonstrate how individual instruments playing in an orchestral piece can be identified and placed in space within playback of a good quality stereo or quadraphonic recording.
Microphones tend to exhibit a linear response over a wide range of frequencies and recordings have to be adjusted to match the logarithmic response of the human ear to sound realistic. Digital filtering and boosting specific frequencies in a recording enable sounds that are normally indiscernable to a human ear to be easily identified.