From what I've heard (this is strictly subjective), the Greddy Evo (I'm assuming that's the model that you are interested in) seemed quite soft compared to other catbacks available for the WRX (again I'm assuming that's what you're asking about). However, it's not as quiet as when it is on other makes, such as a Prelude (where one barely notices that it's there).
Overall, the sound is a deeper tone at idle, a slight rumble, and on acceleration, it's not much louder than stock. It's been awhile since I've heard it, so I can't remember if the unique boxer tone sounds through clearly. Hope you found this subjective audio analysis helpful