By 7 months, infants can tell the difference between different numbers of objects and can match the number of sounds they hear with numbers of objects they see. In one experiment, infants heard either two drumbeats or three drumbeats and then were shown pictures of two objects and three objects. The infants preferred to look at pictures that showed the same number of objects as the number of sounds they heard. For example, if a baby heard two drumbeats, the child would stare longer at a picture showing two objects than at a picture showing three objects.
Starkey, P., Spelke, E., & Gelman, R. (1983). Detection of intermodal numerical correspondences by human infants. Science, 222, 179