Go to program data\electronic arts\ ... \ engine directory.
Rename the 4 car_225 files to back them up (these are the F1 sound files). Copy and paste to make copies of the the 4 car_235 files (these are the Cobra sound files). Rename the copied car_235 files to car_225... to replace the F1 sound files.
This replaces the F1 sounds with the Cobra sounds, but due to differences in the engine (probably rpm range), the sound will be a bit different that it is on the Cobra. In my opinion, it's a big improvement in how the F1 sounds.