Any good horror game is arguably best played in a dark room with headphones on for maximum immersion and System Shock 2 is no exception. With the NewDark patch, it is possible to use a custom sound library which greatly enhances 3D perception on such a setup using Head Related Transfer Functions (HRTF). The library is called OpenAL Soft and it works for all games which can make use of OpenAL, including Thief 1 and 2 and SS2 (since NewDark). Watch the following video for a demonstration:
https://www.youtube.com/watch?v=Yre75KH_Wgw

I will assume that SS2 has already been properly installed and patched with the SS2Tool. Also, disable any custom "3D-stereo" solutions such as Dolby Virtual Surround or CMSS-3D as well as any other globally applied crystalizers or equalisers. Likewise, you won't need any old emulation software like Alchemy or anything similar.

Looking for surround sound support instead? Try this.

• Quick Guide
• General Guide
• Troubleshooting
• Use custom HRTF table
• Acknowledgments

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Quick Guide (System Shock 2, Thief 1+2)

• Download the OpenAL Soft Windows binaries: Download link.
  Copy the DLL from bin\Win32\ to your game's main folder.
• Download the alsoft.ini file from the end of this post and copy it to your game's main folder.
• Open cam_ext.cfg with a text editor and put the following line at its end: 

  snd_oal_device OpenAL Soft

• Consider setting your Windows sound output to 44.2 kHz, which is a perfect match for these games' 22.1 kHz output.

You can play the game now. Make sure that hardware acceleration is set to "OpenAl" and that EAX is enabled in the options menu (default case).

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General Guide/Global installation (all OpenAL capable games)

• Download the OpenAL Soft Windows binaries: Download link.
• Unpack the ZIP-archive, rename the folder to "openal" and copy it to %APPDATA%. You should now have a folder like "C:\Users\your_user_name\AppData\Roaming\openal\bin" and others on your hard drive (Windows Vista and newer). The exact folder setup might be different depending on your operating system.
• Download the alsoft.ini file from the end of this post and copy it to %APDDATA% (meaning, one folder above the "openal" folder)
• On a 64-bit machine, copy the DLL from \openal\bin\Win32 to Windows\SysWOW64 and the one from \openal\bin\Win64 to Windows\system32.
  On a 32-bit machine, only copy the DLL from \openal\bin\Win32 to Windows\system32.
  YES, those are the correct folders, ask the internet.
• Some games like SS2 will let you choose between a generic software mode, OpenAL Soft, or other libraries. If not, they sometimes provide their own OpenAL implementation via a OpenAL32.dll file either in their main folder or their app folder which could be more or less anywhere (Steam folder, AppData, Documents, ...). You'll have to search for it and replace it with the appropriate soft_oal.dll (choose correct soft_oal.dll, rename to OpenAL32.dll and replace).
  You can do the same to the Windows DLLs to eliminate OpenAL from the system entirely, but be aware of the potential danger.
• Open the config tool in openal\alsoft-config. You can define different HRTF tables here but also alter all options not related to headphones since OpenAL Soft supports any kind of speaker system.
• Incorporating EAX2.0+, or even other sound libraries via OpenAL Soft is possible, but more involved. Refer to this post to get started.

You're done, enjoy the experience!

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Troubleshooting
If there is a serious problem like missing sounds or sound not working at all, you've either done something completely wrong (congratulations) or found a bug. The latter can be discussed on GitHub and via the project's mailing list.

If you think something sounds odd or is not working, download the .bat file at the end of the post, open it with a text editor and change the game executable's name to whatever is correct in your case. Copy it to where the game executable resides and start the BAT. Close the game again and post the contents of the soft_oal.log (again, same game folder) here.
Alternatively, watch this demonstration to see - or hear - how everything is supposed to sound in the game.

Other issues:

• The reverb in my game is way too loud.
  Open the alsoft.ini file and uncomment the line starting with #boost. Choose a value that best suits your needs, default is -12.

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Use custom HRTF table
OpenAL Soft allows to create and use additional HRTF tables suited to different head geometries and listening sensibilities, with  KEMAR (MIT) being the built-in default. This video provides a complete listening experience for all freely available HRTF tables and makes it easy to choose the best one for your personal taste. I highly recommend giving it a try to get the best possible experience!
To install:

• Download hrtf_tables.zip from the end of this post and extract it.
• Copy both of the *.mhr files for the HRTF table you picked to openal\hrtf.
• Add the line default-hrtf=<TABLE_NAME> to your alsoft.ini, where <TABLE_NAME> is the name of the file matching your current Windows system loudspeaker frequency. E.g. for 48 kHz, use irc_1031_48000.mhr, then add default-hrtf=irc_1031_48000 (note the missing extension!) under the [general] tab.

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Acknowledgments
OpenAL Soft is a fork of Creative's OpenAL library which adds a host of new features such as EFX (effects library superseding EAX) and the aforementioned HRTF. It supports every relevant speaker format (Mono up to 7.1 surround) out-of-the-box and operates on Windows and Unix-based operating systems. The fork is essentially maintained by one guy who unfortunately can only be spammed via email and does not have a donate button on his website. In any case: All thanks go to Chris Robinson!

this sounds actually very cool, did anybody ever try this? Is it worth the hassle?

Well, I did and I'd say yes. :p

You can look up Hira's YT channel, that user created some demo videos of games which play well with HRTF, including Thief:
https://www.youtube.com/playlist?list=PL_0EBFw_May-IAaplRFVS5n2-9p3pPR_4
Please remember that those videos only really work on headphones.

As for the "hassle" part: You have to download and copy one folder, one file, and edit another one to get it to work in NewDark. That's not much. I'd love more people to try it out and give their thoughts on it. Maybe then, someday, the whole process could be integrated into TFix, SS2Tool and Tafferpatcher to make better sound available to even the most novice of users. There's no reason not to - OpenAL Soft does everything OpenAL can, and better - but without enough people noticing a benefit, there's no point bothering Kolya or voodoo about it.

I'd like to offer a headphone sound option, but generally SS2Tool doesn't screw around anywhere but in your game folder and I think that's a keeper.

It might be possible that the .ini file could also be read from a different folder but I couldn't get it to work the last time. I'll try again some time, might even ask the creator about it.