You can directly adjust the volume through the tablet settings menu.
To access the menu, press on the 6 dots icon :
Then press on the settings menu, a cogwheel icon :
And you will finally be able do change the sound level.
Tip:
You can also just scroll down with your finger from the top of the screen two times and you will see the Brightness Bar and Volume Bar appear. Then you can adjust the sound level directly from that Volume Bar .