In their first weeks of life preterm infants are deprived of developmentally appropriate stimuli, including their mother's voice. The current study explores the immediate association of two preterm infant behaviours (open eyes or smiling) with the quality of a mother's infant-directed speech and singing. Participants are 20 mothers who are asked to speak and sing to their medically stable infants placed in incubators. Eighty-four vocal samples are extracted when they occur in the presence of an infant's behavioural display and compared with random selections during periods of absence of target behavioural display. The results show that infant-directed maternal voice presents more marked emotional qualities when infants display a behavioural change than when infants are passive and expressionless. Specifically, higher values of mean pitch and maximum sound pressure level, as well as greater variability of these parameters are associated with a behavioural display.