Hmmm ... this one's gonna be tough. I'm not really good at cryptograms. Here it is:
"E FM YJA DBFU," QFET KFXXG EY F VJD RJESB, WHXG YJD PHMPEYN AKXJHNK KEM QJ AKFA KB AKJHNKA KB MENKA FAAFSU QYFPB EY F MJMBYA. "AKBY PXJRB EA! MFQABX GJHXQBVW!" QPFA QYFPB.
I think this is the answer, but it looks like the "U" in LOUD was not included for some reason.
"I AM NOT WEAK," SAID HARRY IN A LO[U]D VOICE, FURY NOW PUMPING THROUGH HIM SO THAT HE THOUGHT HE MIGHT ATTACK SNAPE IN A MOMENT. "THEN PROVE IT! MASTER YOURSELF!" SPAT SNAPE.