if someone says something at the top of their voice, they say it as loudly as they can.
'Stop it Nathan!' she shouted at the top of her voice.
at the top of (one's) voice
As loudly as one can. I've hardly slept because my infant has been crying at the top of her voice all night.Sing at the top of your voice so that the people in the last row of the auditorium can hear you!
at the top of one's voice
and at the top of one's lungs
Fig. very loudly. Bill called to Mary at the top of his voice.How can I drive safely when you're all screaming at the top of your lungs?