I'm pretty sure descended larynx is not prequisite for producing vowels. Vowels are merely overtones which occurs because the waves generated in the vocal folds(with a charachtrized fundamental frequency) travel the vocal tract in 3d, which causes destructive interference for most frequencies except those with constructive interference (overtones / formants) When humans move that tongue and lips we basically change the vocal tract shape which changes the overtones, the larynx is behind the lips and behind/in the back of the tongue anyway.