If you're using Safari, then in some cases depending on the browser settings the notification sound will not go off when you have the browser open in the background. You can set it so that the notification sound will still go off in the background as well by enabling the autoplay option in the settings. The step-by-step instructions for these settings are listed below.
- Open the "Settings" screen in Safari.
- Select Website>Auto-Play. With Chatwork open in Safari, you will see chatwork.com in the list as shown below; select "Allow All Auto-Play" from the drop-down menu.