The first Vipers had a funny sounding exhaust from the factory. The new ones that are out now sound a hell of alot better. It's all in putting the right exhaust on there. You can't just throw a couple flowmasters on there with pipes like you can with any V8 and expect it to sound good.
Hell, I've heard 'Vettes with Borla exhaust sound like V6s. 'Vettes with Corsa exhaust sound bad as hell.