Sounds can't be used as a proof that a car was planned but was cut. Let's take MW as example: tvr_cerb is used for BMW tvr_tuscan is used for Clio for_foc_b is used for CTS nis_300zx is used for Eclipse hon_s2k_d is used for Elise fer_360 is used for RX8
and the list goes on... On the other hand, there is a chance that the name in the sound file would stands for a cut car, but I believe that the chances are likely to be very small, or nule. Why? Well, using MW as example, there are cars in the current game which the sound's file name doesn't match with car name. Ex: Camaro uses "for_mus_b" Lancer uses "hon_s2k_e"
Did you get what I'm trying to say?
Now why the hell they have these names, ask it to developers...