Try this real dog sounds and watch their reaction
Get collection of different type of barking, howling, innocent, whining, pug snore, sniffing, angry, puppy and other action sounds.
With this sounds, you can always listen to dog sounds on the go. There are more than dozens of good quality sounds available. It is both entertaining to play with your dog and trick him into believing like there is a real dog suddenly appear out of nowhere.
Watch their reaction carefully as they will be searching for the specific sounds. It can be good for dog training session and teach them by listening to the actions also.
Try when your dog is in happy, active or relax mode. Sometimes they may not react when feeling tired or sleepy.