the sound is not base on the skill, if base on the special effect.
When the special effect of sleep (Abilities\Spells\Undead\Sleep\SleepSpecialArt.mdl) enter the map,
warcraft will automatic play the sleep sound (Sound\Internal\Abilities\Spells\Undead\Sleep\SleepBirth1.wav)
there is 2 way to avoid the sound,
1st way, avoid to use the special effect,
go to:
Objecteditor > Buff/Effect > Undead > Buff > Sleep(Pause) > Art - Target > Remove the path or change to other
2nd way,
go to Sound Editor > Sound > Internal > Abilities > Spells > Undead > Sleep > Right Click SleepBirth1.wav > Replace Internal Sound.
replace with a sound file that got no sound, or other sound u like~