A few months back, Google released Hangout app on Android which replaced the old Google Talk app. The new Hangout app is good, but in some case you might found that you did not receive any notification when new message came.
After some trial and error, I found that it is actually related to the notification sound in which the Hangout app shows that you have selected a notifications sound but actually none was selected behind the scene.
So to fix the issue, here is the step:
- In your Hangout app, go to Setting and select your account
- Click on the notification sound for hangout notification. You will notice that although it displayed a sound to play, when the list of sound is shown, the default selection is actually map to no sound
- Select the sound you want to play
- Repeat step 2 and step 3 for video call as well if you need to