tl;dr they're probably not singing any humanly-intelligible words at all.
You have run into the fascinating phenomenon known as Kajiuran (梶浦語 kajiura-go), a pseudo-language/non-language, named for Yuki KAJIURA, who is the composer for FictionJunction (among other things). One very common quirk of Kajiura's compositions is that they feature lyrics written in "Kajiuran", which is not really a language at all, but rather Kajiura's own term for non-linguistic vocalizations that feature in her compositions.1
The example you cited - the first five seconds of Toki no Mukou Maboroshi no Sora - is almost certainly an example of this. The fact that you can't find any written lyrics for that part is a clue - official transcriptions of the Kajiuran bits of her songs are typically never released, and so they often go untranscribed.
You can find more extensive examples of Kajiuran, too:
- Sis Puella Magica, track #1 from the Madoka Magica OST 1, sung entirely in Kajiuran. Don't be fooled by the Latin track name - it's a ruse! This is a rare case where the lyrics were actually provided with the CD on which the track came (check them out). It's pretty darned clear that this is no language known to man.
- M23, the last track from the Kara no Kyoukai 5 (Mujun Rasen / Paradox Spiral) OST, where all the lyrics are in Kajiuran. This one is more typical in that there aren't any transcriptions of the lyrics, as far as I'm aware.
- Nowhere (live version), a FictionJunction song used as an insert song for Madlax. The main body of the song is in Japanese, but it has little Kajiuran bits and pieces, like at the very beginning and in the chorus.
1 Kajiura has, I believe, stated that Kajiuran is inpired by Latin, though I can't find a source for this at the moment. If I find one, I'll add it.