There appears to be a problem with the exterior and interior sounds being swapped around. The car is louder from the inside for some reason. Also, I am not sure if the car is supposed to have no tachometer, but you can correct me if I am wrong.
The issue is that there is no custom sounds for the car, the closest car to the majority of the Mustangs is the Shelby, using a similar engine to the 428/9 in the two big block cars. And no, the cars did not come with tachs standard.