This articles explains what to do if you're not hearing any sounds from Cozmo.
When you're interacting with Cozmo, there are two place from which you can expect to hear sounds:
If you're not hearing music or game cues (things like narration during intro videos, prompts to play games, Cozmo detecting a cube, and the notes played by Cubes during Memory Match), check the Volume settings on your device (Volume Up/Down buttons) AND the Music level in Settings > Volume & General:
You also should take a look at the device itself to see if it has an issue.
FIX- How to fix no sound coming out from android phone [fixingblog.com]
If you're not hearing Cozmo's voice when he wakes up, snoring when he goes to sleep, or when using Cozmo Says, try increasing his volume in Settings > Volume & General:
If changing the level of Cozmo's voice does not change or increase the sounds he's making, if you still don't hear any sounds coming from Cozmo, please contact us so we can help you further.