Repetition is key. What did you say? Repetition is key. Repetition is a critical component to speech-langauge acquisition. The more frequently something (in this case, speech and language) is repeated, the more familiar it becomes.
Singing songs with repetitive phrases, such as Wheels on the Bus or Old MacDonald Had a Farm is a great way for children to learn. The same goes for books with repetitive language, such as Brown Bear, Brown Bear, What Do You See? by Bill Martin Jr. and We're Going on a Bear Hunt by Michael Rosen and Helen Oxenbury. As tiring and as boring as it may be for you, the adult to read the same book for the zillionth time (I know it's not a real number but you get the point), children learn more and more each time it is read to them. For example, the first time a book is read to them, they may just think "Hey. That sounds kind of fun and I like the pictures." After the second or third time, they may begin to recall pictures that are familiar to them. After several more times, the child begins to recall more details and may even begin to "retell" the story by themselves, as they turn the pages. This, of course, is my favorite step since you get to really hear how it sounds to them.
Similarly, when we repeat sounds, the child gains more and more knowledge about those sounds, as they are heard over and over. When a child first begins to use sounds, they usually say the vowel portion of the word most clearly (for example, "ow" for "meow"). It is usually not until the word has been said many, many times that he or she begins to acknowledge the other sounds within the words.