Hover the cursor over the video you are currently watching to make the video controls visible.
On the bottom right, you should see volume controls.
To control how high or low the volume is, hover the cursor over the volume bar and click the left side to lower the volume or the right side to raise the volume.
If you’d like to mute the video entirely, click all the way to the far left side so none of the bars are highlighted. Click again to unmute.