Click the Edit icon under the video file you selected to open the video editor window. Then go to the Audio tab and locate the Volume option. Adjust the video audio volume to louder by moving the slider bar to the right to make the video louder. Hit the OK to confirm it and back to the home interface.
How do I increase the volume of a quiet video?
https://www.youtube.com/watch?v=243IXfzh0eQ
How can I increase the volume of an existing video?