Your baby hears and recognises familiar voices in the womb even before you give birth. [1] They were able to tune into voices and listen to the tones and speech patterns they were hearing as soon as they were born; this is how your baby learned that words have meaning. Now that they are a bit older, they're starting to connect understanding to the words.
Words are slowly becoming clearer to your baby and your tone of voice is starting to make more sense. Listening to the exaggerated tones, and change in pitch, has given your baby the opportunity to connect your words to not only objects, people and routines, but also emotions, such as love and happiness. Emotional life is mostly about coordinating our actions with those of others [2] and your baby is starting to make sense of how to read and respond to a range of emotions.
When you speak in a soothing tone, your baby will feel peaceful, and when you speak in a joyous tone, your baby will respond joyfully. Your baby is starting to pick up on the changes in your intonation and they can now reciprocally respond to the tones. This is a crucial component in their early learning because it teaches your baby the difference between statements and questions.
More recently, your baby has been watching and listening to you talk. Whilst doing this they are connecting words and trying to make sense of the language that they are hearing. This is called Receptive Language. They will be taking in words, especially when they are associated with objects or routines and some of these words will motivate your baby to respond. If your baby is excited about the words you are saying, their response will usually be a vocalisation, motion or amusing noise.
At this age, it's natural for your baby to understand far more than they can verbalise. You may have noticed that your baby may turn to you when you call their name, or that they may feel upset if you say 'No' in a firm voice. If you say 'No' in a pleasant manner, your baby will understand that the interaction is enjoyable. They're demonstrating that they understand a few fundamental words, but the tone in which they're delivered adds to the significance.
Your baby will begin to connect more words and respond to more familiar objects and people in their environment. They'll keep connecting your speech tone to the environment. Your baby will also improve his or her ability to comprehend instructions and respond to questions. When your baby is able to communicate with more control, they will begin to develop their own responses and behaviours in order to achieve their desires and goals. [3]
References:
[1] Partanen E, Kujala T, Näätänen R, Liitola A, Sambeth A, Huotilainen M. (2013). Learning-induced neural plasticity of speech processing before birth. PNAS. Available at Learning-induced neural plasticity of speech processing before birth | PNAS
[2] Gerhardt. S (2015). Why Love Matters. How Affections Shapes A Baby’s Brain, 2nd Edt. Routledge.
[3] Sample Gosse, H., & Gotzke, C. (2007). Parent/Caregiver Narrative: Interacting 7-9 Months. In L.M. Phillips (Ed.), Handbook of language and literacy development: A Roadmap from 0 - 60 Months. [online]. London, ON: Canadian Language and Literacy Research Network. Available at: Handbook of language and literacy development