According to
http://www.wowinterface.com/download...dforWrath.html there are two level up sounds. Not sure which is the current one. But you could manually replace it on your copy of WoW.
/script PlaySoundFile("Sound\\Interface\\Levelup.Wav")
/script PlaySoundFile("Sound\\Interface\\Levelup2.Wav"
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"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff,
The Tao of Pooh