I have seen some sound files floating around on this website. Here are two...
http://www.e90post.com/forums/showpo...9&postcount=21
http://www.e90post.com/forums/showpo...4&postcount=95
I think it might do justice to call the dealerships and find out which dealer, if any, might have one installed on a test vehicle you could try out. The sound will vary slightly by vehicle as my dealer said it sounds noticeably louder and deeper on my factory engine setup then any other 335 they have installed it on.