Ensure you have the video you want to share open but not playing.
Move the mouse to the bottom of the screen and click on SHARE SCREEN in GREEN
You will see a green and red buttons at the top centre of the screen. Move you mouse over them and the a menu will appear above them. As seen below.
Notice the last item on the right has three dots. …more
Click on that and a drop down menu will appear as seen in the second image below.
Make sure the item share computer sound is selected.
Also on the top menu select mute so it does not pick up your voice.
This makes the sound work properly with Zoom.
When your video is over, click on stop share (in red)