Well, every single room I've assigned some kind of room ambiance, because I hate it when there is no ambiance. Darkfall (by a dev in your country btw) has nothing but music in their game ambiance wise, and that's something that really seems unfinished to me.
After that I pretty much have a sound for almost everything planned. Whether JBurger wants to spend the time programming that in is up to him.
I don't think there really is "over doing it" even with nostalgic games.
Let me give you an example:
In that first scene with the rain, I have a loopable rain "track" that pretty much plays like music. The rain track also has a few things added to it within the track to give a bit more life, like a siren in the distance. I also gave 2 other varieties of rain, so when switching room it feels like your moving. One which was designed with heavy rain, and one which is light.
After that I have the terminal there in the picture make sounds when clicking buttons, as well as a voice over that will talk to you (as if it was the computer) that happens throughout the game. Then I have a sound when you try to open certain things and they do, or do not open, both have sounds.
I hope that makes sense. I don't think your overdoing it.
And I don't think I have you on skype. Mine is sonrise03 (id)
EDIT: I probably won't do footsteps, but we'll see.