Or is here something more what i miss ?
This is the matter of from which end you approach the problem

A.) You may make each of your world's element responsible for generating ambient events. *Even* if only the potentially visible objects can generate the ambient events, you don't have much control in this scenario, because your event generators form a decentralised system. To fine-tune your ambience you need to tune each of the objects that generate the ambient events. Maybe even edit your maps.
*And* it's very hard to balance the processor load.

B.) The EventRain (tm:lol "particle engine" that controls the density and other parameters of ambient events from *one* source.
It's may not be as important for a 2d with limited screen area, but for 3d it's critical.

For example, you can add the ambience zone radius slider to the render quality controls.

As I see it (theoretically) there souldn't be more than a half-dozen ambient events running at the same time (more likely 2..3), and the events must be pretty short.

A good example (not of this approach but of event-generated ambience) is a situation where the passive monsters on the arena occasionally emit a short sound (grunt/screech/breath). Such a simple mechanism but allows to feel what is around