I was recently listening to some music while working on a book I'm writing and an orchestral piece came on. There was a woman singing over this instrumental but it wasn't lyrical words, more just sustained vowels. An example of the kind of music I'm talking about is the song 'Ocean Lullaby' by Henrik Astrom. It is a part of the 'Submersive - Colossal Trailer Music' album (which you can find on youtube). My only way of describing it was like a sirens song but I'd really like to know the name of the technique if anyone has any ideas or knows what I'm talking about.
That's called a "vocalise" (pronounced "vocal-ease")
Vocalise dates back to the mid-18th century. Jean-Antoine Bérard's 1755 compilation L'art du chant includes a selection of songs (sans paroles) by composers such as Lully (1632–1687) and Rameau (1683–1764), chosen for their value as exercises in vocal technique. Accompanying the exercises were instructions on mastering the technical challenges they posed. By the 19th century vocalises were commonly composed specifically for pedagogical purposes rather than being adapted from existing songs.
A related tradition of vocalise sprang up in the 19th century, with wordless technical etudes set to piano accompaniment, following the fashion of the time of setting even the most mechanical of études to piano accompaniment with the thought that this would inspire the performer to execute the music more artistically.
In the early 20th century, many orchestral scores incorporated wordless choruses (especially female choruses) for coloristic effects, and such choruses may be found in works by Debussy, Ravel, Vaughan Williams, Holst, and in many film scores.