This is not supposed to happen, and nobody reported it before. Do you use P3D maybe ? Because there are some problems with AI sounds in P3D, which behaves differently than FSX.
If you use FSX, the problem might be an issue with your audio drivers, than don't properly support some OpenAL features, like distance attenuation, because every vehicle sound uses 3d positioning, and after a certain distance from your eyepoint, they would become too low to be heard in any case, even if we didn't turned them off, which we obviously do, of course.
