In a rather ironic twist, game recording samples have increased in quality (to the point where they're at least 44100 KHz instead of 22050 KHz, not factoring redbook CD audio), but game positional audio has taken a steep nosedive ever since XAudio2 + X3DAudio and FMOD Ex became dominant. I'm still rather bitter about that, especially because we might still have proper 3D audio in games today if Aureal didn't get sued, bankrupted, and eaten by Creative.
As for mods to improve audio, there's an Old Unreal Multimedia Patch for the original Unreal (including Gold) and Unreal Tournament that adds OpenAL and FMOD audio renderers, so you don't need to futz around with DirectSound3D wrappers like ALchemy to get good sound.
The recent patches for Thief 1/Gold, Thief 2, and System Shock 2 also add native OpenAL support. These games need it even more because their DirectSound3D implementation really doesn't like non-hardware audio devices (so Creative's USB audio devices and other software OpenAL devices don't work properly with these games when wrapped through ALchemy).
These community patches won't improve the source audio any, but native OpenAL helps a lot with getting proper 3D positional audio under Windows Vista onward, after Microsoft stupidly axed DirectSound3D.