I noticed that cars don’t sound anywhere near as loud as FM6. I have to turn the volume of everything else to 1 and turn the radio off to hear them. My headsets are maxed out and it’s just adequate… Or is it just me? The music is plenty loud but the cars are way too quiet
And saying “just turn the volume up” isn’t a viable option. Why should I have to turn the volume drastically up and down between different games on Xbox?
Completely agree. The sounds just don’t sound as “raw” and loud as FM6. There have also been issues with the quality of the sounds. There been so many issues with sounds since launch and the only one they fixed last patch was the SV. To name a few: the Supra, R34, 2017 Raptor, GT350, M4, and basically every car that is supercharged! And that is just a few of them! Come on T10, sounds are a huge part of the game.
Never really noticed the cars volume was too quiet but I do agree that there are some sound quality issues with quite a few cars.
Options necessary to Forza Horizon 3.
The more people play Horizon, the more sound threads you’ll see popping up!
I’m not sure if this is a deliberate design choice on PG’s part but the car volume definitely needs to be more pronounced, especially on off-throttle for a number of cars - M4, NSX, GT350, GTR etc just to name a few.
I also feel the game should have undergone heavy testing to make sure that a 2017 Raptor sounds like it should or the Supra, for instance. The fact that sounds are shared across manufacturers and most V6s or V8s sound too similar is a letdown as well. Superchargers and turbos sound a bit mute amd not varied enough, and the absence of upgraded engine/exhaust sounds seems to have carried over from previous Forzas.
Horizon is immensely enjoyable although these minor gripes sometimes break the game quite badly. FM6 was a somewhat positive direction in terms of sound, albeit a few misses.
The real problem is people will continue to buy games like Horizon because they sell like mad! Plus it has little competition, which may not compel/motivate the developers to work as hard on getting car sounds right or resolving existing issues for that matter. Nothing like a little competition to get you off your laurels and strive for perfection! 
Just my two folks.
It’s not very amusing seeing the age of this post and seeing that the problem is yet not resolved. Engine sound of BMW M4 in cockpit and hood cameras is like a tenth of the volume it should have. It would be better that this issue was addressed late than never.