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

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.