I would like to know wen do new born babies start reacting to sounds?

Immediately. Studies show a preference for mothers voice by age 3 hours and dads by the end of the day. These are often a subtle like the flicker of an eyelid.
Baby. Can start from the very beginning, where they have their startle (Moro) reflex. Doesn't have to be to all sounds and reacting all the time.