For those of you, particularly tuba players, who drop your jaw and form your oral cavity so as to say, “hooooo,” or, “huuuuuu,” will find that when you inhale using your nose as well as your mouth, that it will be almost physically impossible to form those vowel sounds.  This is OK.  One should not drop the jaw to breathe, nor should they use those vowel concepts.  The correct concept is, “aaaaaaaaahhhh,” with a relaxed tongue and shoulders down.