In the original Homeworld credits, that song is credited as Angnus Dei, composed by Samuel Barber. There's also an arrangement for string orchestra thats almost as good.
To make sure we're on the same page here, we're talking about the song that plays the first level (training), third level (destruction of Kharak) and 16th level (Hiigara). The one that's solely vocals, and what I consider to be one of the most beautiful pieces of music in the woirld
Both versions that I have are over eight minutes long.