My PSP won't play most of the songs i bought of iTunes??

i got 17 songs off iTunes. i got: almost easy, A7X; beast and the harlot, A7X; *** on feel the noise, quiet riot; evolution and coming undone, Korn; rock you like a hurricane, scorpians; 10,000 Fists, Land of Confusion and Decadence by Disturbed; them bones, alice in cains; bodies, drowning pool; hand of blood, bullet for my valentine; snap yo fingers, lil jon; mother, danzig; bring tha noize, public enemy; youve got another thing coming, judas priest. evolution by Korn is the only one that shows up under that section of the PSP. it is identified as an AAC type music file. wtvr that means