I don't have any experience with the Ixil mufflers, but most motorcycle mufflers need to be repacked at some point. It all depends on how many miles are on it, and how hard the miles were. If it was put through mostly light riding, then you would not have to repack it often. But if it has been ridden hard most of the time, belting flames out the exhaust, you will have to repack it more often.
As for what they are filled with, it's actually a fiberglass material. If you feel like the exhaust is getting too loud for your liking you can usually get some packing material from any bike shop. All you have to do is drill out the rivets that connect the outer sleeve of the muffler to the ends and carefully pull the sleeve off. You will see what I am talking about with the material. Just remove the old stuff, wrap the new stuff around the core as directed on the packing material package, slide the sleeve back on, and re-rivet everything together
Here is a video to give you an idea. Keep in mind each muffler will come apart a little differently.