I wish there were a single answer. There are a few answers that you see from time to time, but I am not sure how good they are:

1. Encourage play and social interaction. Try singing, nursery rhymes and any other interaction that they enjoy. Many kids will gradually associate the activity they enjoy with the sounds. Remember to use words (or a single word) as part of every game. If the child shifts their focus to something else, try to make that a new game.

2. Imitate your child. This helps in some cases. If a child is making sounds at random, and you mimic the sounds, it reinforces the process of making the sounds voluntarily.

3. Use both verbal and nonverbal communication. Each time you play with a toy, for example, make sure your child can see your mouth when you name the toy. Make sure they can see you nod your head when you say "yes" and so on. Many kids build up their receptive language for years before going on to the next step.

4. Use traditional ABA. Whenever we asked our son to talk, we always gave him a few extra seconds to do so. He eventually got the reward (usually a cookie), but he always got it faster (and got more) if he tried to speak. Language came very slow to him, but ABA techniques helped.

5. Simplify your language. Again, kids learn sounds before they learn labels. So give them plenty of opportunities to listen to one-word sentences.

6. Visual Supports. These also work better for some kids than for others. For some kids, the app (or device) helps them organize their thoughts into simple ideas, and then learn the word. Other kids view the devices as just another hurdle in a long list of incomprehensible requests.

We sing all the time around our non-verbal autistic 7 year old. She is beginning to babble again and even sings different notes with an "ahh" sound. All she used to do is just scream but now she will make different sounds. It's not words, but it sounds like pre-speech to me. She may never become verbal, but have hope and try not to get frustrated. She has found amazing ways to communicate with us even though she is not speaking. Have hope.