Where is the sound of flybys? Are they not working properly now?

Possibly. I would have to check. They were re-implemented as part of the new flight model quite a while ago, and then I found a problem with them and fixed that a little while ago, and haven't checked them in a while. I know they've generally got a bit quieter, and it takes a bit more of a crazy flight maneuver to trigger the loudest one, and maybe people aren't just getting up to those kind of speeds in atmosphere anymore and we need to readjust for that.