In the Prisoner of Azkaban Harry Potter falls under the draining effects of the Dementors (or Boggarts pretending to be Dementors) more than once. Each of these times Harry hears voices: his mother's or his father's and (presumably) Voldemorte's.
For example, during his first lesson learning how to cast the Patronus charm he hears the voices twice:
‘Expecto patronum!’ Harry yelled. ‘Expecto patronum! Expecto –’ But the classroom and the Dementor were dissolving ... Harry was falling again through thick white fog, and his mother’s voice was louder than ever, echoing inside his head – ‘Not Harry! Not Harry! Please – I’ll do anything –’
‘Stand aside – stand aside, girl –’
White fog obscured his senses ... big, blurred shapes were moving around him ... then came a new voice, a man’s voice, shouting, panicking –
‘Lily, take Harry and go! It’s him! Go! Run! I’ll hold him off –’
The sounds of someone stumbling from a room – a door bursting open – a cackle of high-pitched laughter –
‘Harry! Harry ... wake up ...’
[Prizoner of Azkaban, UK Hardcover, Ch. 12 The Patronus, Pg. 177 & 178]
What causes Harry to hear (or remember) the last few moments before his parents' deaths when the Dementors overwhelm him?