Although one might think all slip-ons sound alike, the truth of the matter is that they really don't. What makes an exhaust system sound louder from one brand to another, is how the muffler itself is constructed. Using larger diameter tubes and whatnot (in example a full system) helps a little, in terms of a making the system louder but its the design of the muffler that gives each system its unique sound.
What factors determine sound, you ask? How the muffler is chambered, or not. The more chambers (passages within the muffler the exhaust has to travel through before being released into the air), the lesser the sound. Is the exhaust baffled (insulation like material packed internally into the muffler to help absorb sound). How much or what areas of the muffler is baffled can effect how deep the exhaust note is. Then we have straight though designs, which is exactly that. The exhaust passes straight through a perforated / baffled metal tube. The length, diameter of the tube and the amount of baffling around will determine its exhaust note. Whether or not cat's are present will also make a HUGE factor in how "loud" the system can be. Last but not least, the way the exhaust gas exist the muffler / system also plays a role on how the systems sounds. The same way a megaphone amplifies sound, if the tips are shaped in a similar manor, the exhaust note will be increased as well.
One more thing to consider, different systems will sound different at different sections of the RPM band. You may run into two systems that sound alike at idle, but under hard acceleration one may be significantly louder than the other.
Sound clips are a good way to ballpark how the system may sound on your bike, but there's no way, even a good microphone will be able to pick up the low gut wrenching resonance which is what you need to not only hear but feel when picking a slip-on system. Best advice I can give is to actually see and hear the system in person before you purchase it. Even if you cant hear it on you exact bike, quite often specific brands manufacturer their mufflers in the same way, so although it may sound slightly different from model to model, the general exhaust tone will be easily distinguishable.