What is this mono thing with GT4 you are refering?
I'm guessing there's some interference between the stereo channels, resulting from playing two identical sources with one (or both) of them shifted slightly in "phase" - basically, the illusion of Stereo is in many cases created by simulating the inter-aural time delay by delaying the two channels differently (either in the software / game, or because of speaker location.)
In reality, there is only one source, and each ear hears a different
colouration of the same sound (as opposed to a delayed version of the
exact same sound.) See [WIKIPEDIA]Head Related Transfer Function[/WIKIPEDIA] for how the "colouration" can be achieved in-game. The main points of interest are the shape, size and relative location of the pinnae and "head shadowing" effects, the latter having a filtering effect.
Check out pictures of
interference patterns, created by coherent (identical) sources in different locations to see the kinds of "damage" to the waveform that occur, the effects change depending on where you listen from (it's this phenomenon that allows you to find a "quiet spot" in the pattern created by a fire alarm sounding in an enclosed space, simply by moving your head about... although in this case, there may only be one source, but the reverberation creates this interference pattern.)
Listening with headphones should eliminate the problem if it's only due to speaker location; if the delay is (also) imposed in software, then you'll get some interesting binaural effects, assuming the channels are identical aside from the delay (and volume, naturally.) "
Binaural beats" are an
example of how our brains treat each ear separately. Listening to each channel in isolation (removing the other earphone) should sound like a pure tone, whilst the combination "wobbles" at the same frequency as the difference between the two separate tones. This can have some interesting spatial effects, given this is the sole reason we have two ears...
I haven't tried what CoolColJ mentioned, but it makes sense; listening to off-screen footage of GT4 shows some serious "echoing" going on (most noticeable in mono
recordings of stereo+
speaker output from the game.) I think GT5
does the same; it sounds better to me through my headphones than my 5.1 setup (less muddy / washed out in the midrange, better localisation) - but I do have a problem with reverberation, too.