If you would like to lower the volume of the game announcements or dings, choose "Game Volume" under the options menu (the gear).

The volume dialog displays. There are 2 separate controls: one for the game announcements and one for the dings (indicating your turn).

By default the volume is at 100%, but you can lower it by dragging the slider to the left.

To mute the sounds completely, drag the slider all the way down to 0 or click the mute button on the left.

Note: this only affects game sounds and not the volume of other players' voices. If you wish to affect all sounds, please use the volume control on your device.

Tip: make sure your sounds are not muted in any way (including "do not disturb" and other similar modes)