:\ If she can still talk high pitched [but we rarely see her do that], she can probably sing that way too.. I think it was shortly after the coming of MY STORY that Ayu sort of stopped using it all together xD
High pitched Ayu makes me think:
'Ayu Coooookie tiiiiiiiiiiiiime~!'
I really don't think that she is unable to use that high pitched voice anymore, but rather she is just holding it back.
Eh, we'll never know anyway XD