Playing with sounds ensures that your child hears these sounds and will be able to imitate these sounds when they are ready. Sitting in front of your child or lying them on your tummy while lying flat on the floor will encourage them to look at your mouth and therefore imitate sounds that you make as they get older.
You may feel silly repeating these sounds, but these are the beginnings of speech sounds and are important for your child to imitate as they get older.