ZS-NDV,
1.) First point is valid. Completely valid. I have flown some old dogs and I know at best of times it's impossible to hear yourself chewing on food let alone breathing!
2.) Regarding the altitude selection. I dunno, some a/c have knobs which do have the faint click click sound, other's don't but it doesn't matter as the altitude selection on the MCP was relayed via Mode-S.
Even the anoraks picked it up
Putting 2 together from here and 2 from there results in me being convinced the guy just wanted to end his life in the most spectacular and selfish way possible.
I would highly recommend a review of
this article for all.