Speaker on iPhone not working with Skype or Viber
This is a member of the Viber Development Team! :)

Please see our Help Desk at: http://helpme.viber.com/index.php?/Knowledgebase/Article/View/25/0/iphone-viber-connection-issues  for information concerning connection and audio issues.

If the issue persists it might be related to your own specific device. (You can contact Apple support).   

If you have any question about Viber - please feel free to ask.

Thank you, 
Viber Team.