I got the white cars too after a recent reinstallation of the game + patch + nocd.
This was due to the fact some values were left from past GP4 installation in windows registry.
So when I installed game it was ok, when I installed patch, it seems to install something but warned me it was already installed (so it wasn't installed), and thus I installed 1.02 nocd over a non patched game, resulting in white cars for half of them.
So 2 things to check :
- did you use the right nocd ? 1.00 for original, 1.02 for patched one. Maybe in your case patch worked but you installed 1.00 nocd over patched game.
- uninstall your game, go in registry using regedit and check and delete if present : HKLM\SOFTWARE\Wow6432Node\Infogrames\Grand Prix 4 hive (folder). Note that this is the path on windows 7 64 bits, it might be different with another OS.
Then reinstall your game, patch and nocd and it might be good for cars.
For sounds, what's your sound card ? I believe you might try to reinstall your sound card driver. I got different troubles with sounds too and it was always due to my sound card driver. Damn Creative drivers.