It is also possible to use Sound Editing Programs for 3D Sound.

Using Volume Automation on the Individual Left and Right Channels.

So increase the Volume gradually on the Right Channel while slowly decreasing the Volume on the Left Channel can really help with the Illusion.

I did that on my 106 Ambience Track with the Footsteps Sound.

Though, A Bi-Aural Microphone may produce more realistic and better quality results.. Volume Automation can be used as a back up for those who don't own such Microphones.

Mega-bump, because this is rather relevant. As FireFox pointed out, sounds in the game can, and a few are written for both channels. I went and made a few of the pre-existing ambient sounds stereo, so they can possibly be used as other variants of the current ones. You can listen to them here: https://soundcloud.com/redacted-2 . Somehow, ambient6 never fails to send shivers down my spine, every time.

Omniary, those are just too awesome for me to describe.

https://soundcloud.com/redacted-2/096-guard
Especially the 096 Guard one, it made me do this: EDIT:
I found the 106 spawn too:
https://soundcloud.com/redacted-2/106-spawn
I would love to hear that in the actual game