Unless your mom signed for that letter I would not respond. Other then this thread saying you recieved the letter and your visit to their site they have no proof you ever read it. The entire process of sueing people for copyright infringment has more to do with mass mailings of extortion letters then actual legal processes. They make money on the few unlucky people with thousands of songs that put up a fight, the masses who settle out of fear, and basically anyone who responds.
You think they are actually going after you? They are going after every single person who used that p2p program- all at the same time. One or two lawyers work 40 hours a week sending out these letters to anyone remotely evident of copyright infringment and the 20 or 30% that settle immediately pay those wages and the loss of profit the thousands of millions of downloads cost Sony.
They say you DLed 8 songs and are charging you 4000$ thats 500$ a song. Or roughly 500 songs DLed legally. If even 1/500 or .2% of illegally Dled songs are prosecuted at 500 times the cost of a legal download SOny breaks even (minus the cost of lawyers and politicians and judges.)
Basically- ignore that shit, don't fight it- it will draw attention. Eventually at some point they will give up or you will be summoned to court. If the later happens, buy the **** CDs, loose the reciept and prove you own the songs already and your DLs were perfectly legal.
Thats what I would do- especially since there is no way for the RIAA or any agency or person for that matter to sue you without you going to court in person.
Also jeep in mind I have no experience in this matter, or anymore then a basic knowledge of common law.