I had a scene in a short film I did last year where I did something fun.
The film is a period piece, so no modern sounds anywhere. A woman is sitting alone in a room contemplating a potion she's in the process of making that will kill her unborn child - saving it from her abusive husband.
I put a clock in the room (even though there wasn't one anywhere on screen in the set) and had it ticking almost inaudibly at first. Just part of the furniture.
The woman is interrupted as husband storms into the room, threatens her and leaves. he's been doing this for years.
I do some other designy things around her state of mind there (stretched screams, some forgotten zither things, etc), and in a fit of rage she makes the potion strong enough to both kill herself and the unborn child. the camera starts spinning around her head in closeup and I take the clock and push it very loud in the mix - just a massive ticking thing signifying the last moments of her life. The audio and visuals are chaos for a moment as she's going to through the decision making process that will lead her to drink the potion.
then a hard cut to a wide shot of the woman. she's made her decision, now she just has to do it. The clock is back to its regular place - its there, but just part of the furniture again. The rest of the bgfx are back in place. wind, house creaking, etc.
she takes the cup and lifts it.
kaboom!! - a piano that some movers were lifting in a previous scene crashes through the window.
The cup falls and shatters. The potion is ruined. The woman is disoriented.
At this point I got rid of all of the bgfx that weren't a part of the piano ringout (thanks to Tim Prebble for tortured piano http://hissandaroar.com/sd006-tortured-piano/ there) - including the clock. Its a subtle thing, but that decision was a specific metaphor for the fact that the seconds are no longer ticking down on the woman's life.
The movers walk into the room and address the woman, but we're still only hearing the piano breaking apart on the floor and the dust settling. Despite all of the visual motion, all of the audio is of the event settling into the ground.
no clock, no wind, no house creaking, no men talking even though we see them addressing her and helping her off the floor. Just the long ringing dissonance of the piano as it lays there and the falling of dirt to the floor and the occasional random string pop as it finally gives way. The scene settles into quiet and then hard cuts to the woman screaming as she gives birth.