So, I had the same problem and took a look. I actually found a solution:
DriveV seems to override the vehicles.meta. For the police (stanier), police2 (buffalo), and sheriff (stanier), it sets <audioNameHash> to GLENDALE and BUFFALO2 respectively. Those are civilian types. The other police vehicles do not have that flag set (<audioNameHash />).
So, to fix it, open
mods/update/update.rpf/common/data/levels/gta5/vehicles.meta
and set
<audioNameHash>GLENDALE</audioNameHash>
under police and sheriff to
<audioNameHash />
This way it will use the default sound for the car category. Do the same for buffalo2 and it should work