This sounds very much like a local issue. Voice lines are strictly mono - they're all coming through a radio after all, assigning them a position in the stereo field wouldn't make sense - but while the sound effects are monophonic like normal they're definitely played back stereophonically. If you're not hearing that then the issue might be to do with your drivers or audio setup.

EDIT: ...never mind, looks like your edit was while I was typing. I'll leave the reply there in case someone else has this :)

It's been 21 hours since I edited that post. And I thought I was a slow poster, lol.

Anyway, something is definitely off about the audio positioning and what is described in this post on the Steam community certainly suggests that it's not just me, that the audio positioning is kinda half-assed and there was indeed that IAS mod that improved the audio positioning on A3D-compatible hardware. As is the positional audio really is a lot worse than in other 3D games from that time.

And since you mentioned the radio messages: besides the positioning of sound sources I do actually have some more issues with the spatial audio in this game (but the radio messages are indeed okay).
1. Your own engine sounds should be stereo. I love the engine sounds but they really eat up all the other audio, being this constant aggressive humming dead centre. Replacing them with stereo sounds would have not just made the overall sound less dirty, it would have also very much improved the feeling of being inside those vehicles.
2. The soundtrack has a really bad mix. It seems that the individual instruments are all mono and most of them are played in dead centre again, with just the occasional one being played with some panning. That really doesn't sound good and further ruins the overall mix. It's worth mentioning that the Red Odyssey soundtrack does not suffer from this issue.

All of this gives the game a really bad overall audio experience and is in my opinion stuff that should have been fixed in Redux.

It said 17 minutes when I posted, but... I dunno, maybe the website glitched out on us.

When Battlezone released it was very much trying to show off its hardware 3D rendering - it could be that updating the audio engine fell by the wayside as a result (Battlezone's engine has a heritage that goes back to MechWarrior 2). Oddly, despite having studied sound design I've never looked too closely at the games I play so I don't fully understand how you can do badly at what I'd have expected to be the simple and purely mathematical operation of resolving the angle of a source from the viewer's facing direction to a component of left and right - maybe I'm just spoiled by the modern sound engines and middleware that handle all this natively.

Regarding stereo engine sounds, I can both see where you're coming from and see why this might not have been done if it was a deliberate design choice. Too much stereo information could run the risk of confusing the user and having them mistake a left-hand-side peak in the engine sound for the sound of another engine to the left, which whilst a pretty minor problem could grow to be annoying fairly quickly depending on how frequently the engine sound loops - and could risk the player ignoring an approaching engine from the left if they're not paying attention (though this is Battlezone, not paying attention gets you killed anyway). It's not a good reason to keep to mono engine sounds, but it certainly is one.

Music-wise, I think that was probably just the nature of Jeehun Hwang's workflow. He does have a very identifiable sound and aside from instrument choice I think his mixing is probably one of the reasons for that (I'd point out that you'd normally record real instruments in mono anyway, but I guess that was probably a slip of the brain :P ).

I'm glad you agree radio messages should be mono, I'd quite literally fight you over that.

Just to be clear, I'm not hating on the developers and composer for what they did at the time. They mostly did a great job although worse than some titles at the time. My concern is only with the effort put into the Redux version where audio, unlike the graphics, seems to have remained entirely (or at least almost entirely) untouched and really stands out as something that is sub-par.

And I remember being blown away by the engine sounds at the time. It's just stuff that could and should have been improved in Redux in my opinion. I don't really share your concerns on a stereo engine sound when in-cockpit, I think that on the contrary, a different and stereo sound for your own engine could have made the impression of being inside that vehicle far more impressive while being also less aggressive and easier to tell apart from other vehicles, cleaning up the whole mix in the process. But oh well, we'll never know, will we.

As for stereo instruments:
Well, that's a topic we could discuss at length but not all instruments are recorded in mono and even then, you'd usually slap some stereo effects on them that give them some width. And especially orchestral performances are recorded in their entirety rather than slapped together from isolated mono recordings and any remotely modern orchestral library provides instrument and section samples as stereo.

Anyway, my main issue isn't that the instruments themselves are mono, many soundtracks from that era sounded so "dry", it's really that they are too densely placed at the center, which, in combination with the aggressive centered engine sound and weird panning (and also probably too quick fall-off for external sound effects) gives the game a pretty ugly, kinda "muddy", overall sound.

And I should mention that I'm actually quite a fan of Hwang's works, I love the Mechwarrior 2, Dark Reign and Heavy Gear soundtracks and curiously they don't suffer from that issue I mentioned, at least quite as much.