You can adjust the notification sound that is made when a new message arrives, as well as its volume.
1)
Clicking on [Your name] in the in the upper right of the chat window will display the menu tab [Preferences] → [Notifications].
2)
Please select your desired notification sound from [Select sound].
Confirm the notification sound by clicking [Play].
3)
You can adjust the notification volume to the desired level with [Volume].
4)After setting the volume to the desired level, click the [Save] button on the bottom of the page.
Note: In case you prefer no notification sounds be made, deselect the [Turn on sound notifications] checkbox.