your voice.. lack of practise. if you do practise, then it's not enough hehe.
basically like what others say, the "flatness" , I only heard your tina arena - burn cover, it's not an easy song but it's a good song to practise with as there's "low" and "highs" in that song and the feel of the song from "upclose-and-intimate" verses to the climax chorus asking ppl to burn. (just nice 7th month. j/k) .
so you might wanna practise with a tuner , and practise the song over and over on the high notes, you often go flat after singing "I want you to burn (high)" then "like a candle in my life" (low = flat) , so practise sing high and low til you sorta "seasoned" extending both range ends of your vocals. you can take it slow, literally. that means you sing this "burn" song at twice the slower tempo, get each word's pitch right first (until all of us can fall asleep) , then slowly speed up as time goes , experience comes.
also it does sound like you're shouting to get the pitch right. practise everyday and you won't need to "shout".
one more thing, as a vocalist (i'm not sure if you're covering the songs for fun or not) , but when you're armed with a mic, you might wanna practise pulling away the "microphone" (in your case your hp) so it doesn't distort too often.
you can learn more from here :
http://soft.com.sg/forum/all-about-singing/67199-vocal-eq-settings-soundman-communication-2.html
and the other threads