Your baby’s body and the sounds that they make
The sounds that your baby can make are partly limited by the size of their throat, so as they grow so too will their range of sounds.[2] Their sounds now are also limited by the control that they have of their mouth, lips, tongue and throat; speaking involves positioning the lips and tongue and controlling breathing and the sounds made by the voice box (larynx).[2]
Your baby will move their mouth, lips and tongue to explore the sounds that they can make and develop the muscles that they need to make speech sounds.[3]
Ways to support this exploration
When your baby is awake and calm, give them time when they can explore moving their mouth, lips and tongue – they might sometimes do this when they put their hand or a toy in their mouth, but also when they don’t have anything in their mouth at all.
If your baby has a dummy, give them times when they are awake and not with their dummy so that they can move their mouth, lips and tongue in different ways to investigate and develop the movement of different parts of their mouth.
You can also encourage this early exploration by responding to the sounds that your baby makes so that they begin to learn that speech is important in building relationships, and to understand turn-taking in conversations.
In time, they'll start to vary their noises, pitch and tone
Over the months ahead, you might notice that your baby makes sounds such as blowing raspberries, or that they make bubbles with their mouth as they explore and develop these movements. Your baby will gradually develop control and be able to make a variety of sounds and vary the pitch and tone of their speech.
They will begin to imitate the tone and pitch of the sounds that they hear and will gradually make a wider range of sounds themselves.
References:
[1] Owens, R.E. (2001). Language development: An introduction (5th Ed.). New York: Merrill
[2] Polka, L, Rvachew, S & Mattock, K. () Experiential influences on speech perception and speech production in infancy. In Hoff, E. & Shatz, S (ed.) Blackwell handbook of language development. Oxford: Blackwell, pp153-172
[3] Moulin-Frier, C., Nguyen, S. M., & Oudeyer, P. Y. (2014). Self-organization of early vocal development in infants and machines: the role of intrinsic motivation. Frontiers in psychology, 4, 1006. https://doi.org/10.3389/fpsyg.2013.01006