Lyrics? There are none, as that's purely instrumental. Onomatopoeia? Assuming it's a pure sine wave, probably just an "ooooooo" without any glottal plosive at the beginning. A pure sine wave wouldn't be a "woooo", that would be a sine wave with some kind of envelope filter or something at the beginning of it. It wouldn't be "ooooOOOOOooooo" unless there's an increase in amplitude in the middle or like, another additional sine wave of a slightly different frequency than the first and going in and out of phase with it or something along those lines. An "iiiiiiiiiii" sung in a head voice might be appropriate if the sine wave is really high in pitch.