I wonder though, is your output one mp3 with all kinds of sounds mixed together,
Why premix? Any decent engine could mix sounds (and you need something to fill up that 32-sound limit of OpenAL, right?). The idea was that the ambience is generated on the fly, from a bunch of short sound and visual effects scattered around randomly.

or are you offering a complete environment with variable inputs, so I can add that to my engine?
It's just a general idea, to implement by yourself (it's more effective that way).

And no, I didn't mean "sounds only". The "decorative object" is more general thing, including such things as particle fountains (could be seen over lava in Quake2), smoke puffs (for dirt/desert), etc. I think, implementing a library of these should be very simple -- much simpler than trying to put these details into your levels manually, on a fixed basis.

Also, the sound library, IMO, is better stored as a set of WAVs, not MP3s -- because the sounds would be short (generally), and there would be a whole lot of them.