There are none mentioned on the liner notes, written by the composer.
The Wikipedia article (type in the name of the piece in the Y/A searchbar), lists the orchestration;
"The ensemble is a mixture of double woodwind, double brass with one tuba, 3 percussionists, 3 wordless female singers, and 2 pianos."
The composer thought of the singers as the sirens of ancient myth, and like Debussy did in his Nocturne, Sirenes, gave the women only vowels to sing, no actual words. (in the Debussy score, the choice of which vowels are to be sung is left to the conductor, this is also true for the wordless chorus in Ravel's Daphnis et Chloe.)
I only heard vowel sounds when listening to the recording conducted by the composer, so this could be a mistaken notion or an 'urban myth,' like, "if you play this song backwards it says 'paul is dead.'"
It could be interesting to learn what people Think They Hear....
best regards, p.b.