A little background: Auditory Depth Perception has been discussed on the blog in a fair amount of detail. So, I'm just going to do a quick recap of that information. Major factors that effect that perception in no particular order: 1) amount of reverb 2) timing of early reflections and their volume/tone 3) volume 4) amount of high frequency content.
1) The greater the quantity of reflections, the further the object is relative to the other objects making noise.
2) The closer the earlier reflections are to the original sound, the further the object is making the sound. See this initial time delay gap demo for a clear picture: http://www.syntheticwave.de/ITDG.htm
3) Softer(quieter) sounds appear more distant.
4) High frequencies are the most easily absorbed. A more distant object will sound more muffled.
How to implement this:
Place the least amount of reverb on the nearest object and conversely the most on the most distant.
Adjusting the EQ for more high frequency(or filtering less high frequency out) may make it sound closer.
The more distant the object, the less 'pre delay' the reverb should have. Different reverbs may have a different name for that parameter, but I think that's pretty standard. That's just a measure, usually in milliseconds, to describe when in relation to the original sound the reverb will start. Sound travels at roughly one foot per millisecond.
More distant objects should be slightly quieter depending on the degree of desired effect.
I like to overdo the degree of these variations on the signal b/c the listener has no visual cues to help them out.
My best example for a window view on a pseudo recorded performance and space(Pardon the vocal):
Everything That I Am by dantheman-10
Obviously I'm no pro and this sounds pretty rough to me right now. None the less I think it gets my point across. I think it's fairly realistic, but not 'real.'