Also today I noticed I kept getting this weird feedback when there were 16 ppl in the room. It was coming in through my Turtle Beach DPX21's and all I could do was turn the sound down all the way or unplug them (it wasn't anyone else as I muted everyone). This has never happened before and worries me a bit.
...And yes cache has been deleted grrrrr...
I, too, have encountered voice problems with my headset. I attribute them to- a: PD servers being tied up for whatever reason b: the host chose a better-than-recommended quality and c: out-of-region players in the room.
a- PD servers haven't been the greatest since day one, but to be honest they're also asked to do an awful lot.
b- Chose "High" when "Poor" was recommended. New player, so go figure.
c- This problem is normally absent so long as everyone in the lobby is of the same region. This isn't always the case, as some countries have a better speed than I do, so I'm forced to make judgement calls when hosting.
For the sake of stability, I generally host with Voice disabled. I don't like it, but it has helped, especially since I apparently have a penchant for racing people with wheels (I use a DS3). See the Bug detailed in the News section. I also limit my room size to 12, although that apparently is becoming too much and I should probably scale down to 10, which I hate but it does provide better racing.
Also, before beginning the race, have all drivers exit their practice laps, so that they're in the pits when the countdown starts, or before 15 secs. left to go in the countdown at the latest. Doing so has helped the transitions from countdown to actual race-start countdown (i.e. 3,2,1, etc.), dramatically shortening the black screen between. Problem with that is convincing 9 other people that it's in their best interest, as you inevitably come across people that want to do things their way, consequences be damned.
As with everything in Gran Turismo, your milage may vary.
Edit- One thing I also forgot to mention is to disable any custom soundtracks. If you're playing your music, or have it enabled, it has effected game play in the past, and (to my knowledge) hasn't fully been resolved. Besides, you should be racing, not headbanging to Boheimian Rhapsody.