This post is for Thief Gold and Thief II, not sure about Thief III

It is of note, for those that have the Logitech G930 wireless headset (and I'm guessing the Logitech G35 as well),
you will have degraded sound performance. Your sound will be restricted to mono in the pure version of these games, no matter what you try with your Windows sound mixer or the Logitech panel. Like you, I was depressed when I first fired either game up. Good sound is key to this game!

For instance, even though you can switch to stereo with the switch on the headset, the Thief series of games will not recognize the change. You also cannot lower the number of speakers the headset emulates in the Windows sound mixer: It always shows up as a 7.1 device. This is true for Windows Vista, Windows 7 and Windows 8. It's a hardware compatibility issue with these older games.

What needs to be done for both Thief Gold and Thief II is to update/patch the games using the (game) community patches that are available online. These patches not only fix and enhance many other things in these games, but they also allow the use of OpenAL.

OpenAL is basically a fix/workaround for people using creative labs sound cards/devices in newer versions of Windows (Vista on up). Installing OpenAL essentially made their devices compatible again. It just so happens that it also fixes these USB Logitech headsets.

So this is what you need to get your headset to work (depending on your version of Thief):

Either
The community patch for Thief Gold - currently called 'TFix v1.13.exe'
or
The community patch for Thief II - currently called 'TafferPatcher_TMA_207_beta.exe'
and
(for both versions of Thief): You'll need the 'OpenAL Installer for Windows' (called 'oalinst.zip' from a creativelabs subsite)

You can find these files by using your favorite search engine (or search the forums here), they will come up.

So, how to do it:

1. Install your GOG Thief game from scratch.
2. Install the appropriate Thief community patch (whichever it may be).
3. Install the OpenAL Installer for Windows
4. Run the game, go into Options, Audio
5. Click on Hardware Acceleration until you see OpenAL.

Enjoy the game! I've noticed that switching to stereo using the button on the headset will separate the sound more than leaving it on surround sound. Choose which one you like more and have at it.

Also: I've noticed you can get a nice echo/reverb effect reminiscent of the EAX games (as well as a great sound boost) if you install DFX audio enhancer. I think there is a trial available, but it's mainly a paid application. Works with everything else in Windows too, and is configurable.

Hope this helps someone!