As far as I'm aware there are 3 production types for the 260.

Stock units. come with the car etc
MG Xpower back boxes. MGs loud version. very loud.
or a set of Zeros. not as loud as xpowers, but louder than stock.

Although there are also cars with custom systems attached. and there is even one without any silencers at all isnt there??

I dont have time to find all the links right now, but there are videos on youtube of stock and xpowers revving and i think Tim has an MP3 of zeros revving.