I've twice had the sound disappear from my game entirely while driving online at Mount Panorama, the first time it lasted for two hours then froze my game while taking the left turn before the second checkpoint and turning my wheel all the way to the right. I was driving the 15th Anniversary GT-R GT3. It's a digital copy from the Canadian store courtesy of a good friend, if the version is important.
It is pretty extreme with racing brakes if you haven't tried them yet, at least on lower grade tyres. Not really what I'd call a glitch though.
@mistersafeway - yeah I've had the sound drop out of my '96Nismo GT-R and my Team Phoenix LMS Ultra R8. I've either had to slightly bump a car's bumper in front of me (or get slightly bumped from behind) to have the exhaust note kick in. Sometimes it would inadvertently come back in after hitting gravel or drifting through a corner causing a lot of tire screech. I have the utmost confidence that PD will have this worked out soon as they usually always do.