I gotchya....I had to get a new BZR, but already had the Toyota and Scion versions, so I tested all three: bone stock, one exhaust or turbo upgrade at a time, and a combination of those upgrades. The Subaru is the only one I could replicate what you're talking about, and it only happened with turbo equipped. I took the Subaru a couple minutes deep into Nurburgring with each test to see if the sound would fully integrate like I expressed had happened in several races with various cars, but it never did. I believe the engine/exhaust note is actually there, but highly muted for some reason. I say this because I use a DS3 and use the downshift a ton when approaching a corner, and the note was there, but just barely. The rest of the audio output from the car was all a steady rush of air not influenced by speed or RPM and that awful tire screech. The Toyota and Scion seemed perfectly normal in this regard, though I must say the sounds all around were most uninspiring. A shame, that, because the Subaru sans turbo was easily the most desirable of the three, and I see why you might wish to have this pepper of a car fully upgraded as it was quite fun. And I did think to check the guage displays of all three, and all three without turbo upgrades showed no guage for turbo, so there's nothing there to maybe guess a screwup happened between different groups of coders programming the game by standard equipment. So yes, you seem to have caught a full-on glitch to add to the pile!