As Toronado said, I think the biggest killer of turbo sound from the in-car view is the racing tranny. The 'High speed rumble' is a pain in the butt also. Go to the behind car view and you can hear the blow-off valves a bit better.
What I find amusing is that in GT1 they had the high pitched whine of the turbos, but no BOV/air 'whoosh'. It was awesome sitting on the line and listening to the turbo/s wind up. Then in GT2 they ditched the turbo whine and added the air 'whoosh' instead. Sounded a bit gay to me, but oh well. The BOV sounds were pretty good - loud too. Then came GT3, and I asked the same question as this topic... "Where did the turbos go?" I mean, some cars (Toyota GTOne for example) had great turbo sound, but most of them were pretty pedestrian. GT4 seems to be more of the same of GT3... as far as turbo sounds go.
What gets me is not so much "where is the turbo sound?", but why are the different sound levels so wrong. You get into a high performance car that has been fully lightened (i.e. NVH padding removed), and all you can hear is engine. It shakes the whole car - rattles through the body and the chassis... Even with big subs on GT, the effect is not the same. The sound is so... anaemic. I do sympathise with those who miss the thump of the V8's in GT. If you've never driven a hard tuned V8, you won't know the sheer presence of this snarling, spitting thing the other side of the firewall, trying to twist the car in half. It's quite an atmosphere.
Even with turbos - you get in a car with a big turbo kit, and you know about it. On boost you can hear the turbos whistling (even through the NVH padding), and the engines just let loose this wicked growl. But the big thing is just the sheer volume of the turbo system - the turbo whistle, the air rush, the blow-off valves. *Sigh* Damn it! Now I wish I was out driving, and not sitting here on my butt, pretending to work but spamming GTPlanet.