Unfortunately a lot of add-on cars for SCGT have this problem.
Sometimes it is caused by installing the cars incorrectly. Check the
zip file you downloaded. If there are .wav files in there, you usually
have to put them into the SCGT\Sounds\22k16bit directory (unless the
readme says otherwise). Unzipping the sound files into the Patch
directory along with everything else will not work. If that isn't the
problem, you may need to edit the .veh file in the Patch directory.
Sometimes the guy who made the car specified a non-standard sound in the
car file, and then forgot to include the sound in the download. Or he
may have the sounds in a separate download on his website. Check the
website where you got the file to see if there are optional sound files
available. If not, open up the .veh file in a text editor. The
filenames that are used for the sounds are usually near the top of the
file, in the [SOUNDS] section. Find an appropriate .wav file in the
SCGT\Sounds\22k16bit directory and substitute the name of that one for
the one you are missing.
HTH,
Haqsau