I'm not being defensive! I was trying to explain the Apple way of letting developers to add there own sound preferences! If the developer doesn't included eat a user wants then find another and complain to that developer.
If you want Apple to add a 'system sound mixer' then report the request to Apple OS X Feedback. If enough requests are received by Apple then you former Windows Switchers might get then to add a requested feature! Just remember the old saying "you catch more flies with honey"!
Thanks for providing a link to give Apple feedback. In my situation the 'developer' is Apple. Apple's Messages and Mail apps are the two programs I'm trying to individually lower the volume for. I use a background noise generator to drown out chatter around my workplace. To do this effectively requires a lot of volume which then creates very loud alerts from Apple's apps.
For the record, I've been a dedicated Mac user for over a decade and didn't even realize Windows had this functionality until coming to this thread.