Actually Milli yeah they do, some do sound awful and have done for a long time. I've played the games on many different pieces of hardware and the fact is you should get better sound on a high quality surround sound system with independent bass BUT it should not sound as tinny and synthesised as it does on a standard TV which to be fair these days have decent sound setups out the box.
The game doesn't sound that great and any idiot can see that, the cars he gave 5 Stars to I was like what? The Gallardo especially is stringy sounding at high revs in the game, plus despite him having a setup with bass it was clear the real world sounds were more throaty and had more bass compared to the game.
You record a video of a car in real life it will sound good on your TV, it will sound even better on your cinema system, but it will not and should not be like going from hearing mono sound out of one speaker to basically being in front or behind the car. Sound systems improve the sound especially the bass, they don't completely revolutionise it.
Forza sounds great on a TV and even better on a sound system which obviously enough is how it should be.