If you have an Apple computer you have access to GarageBand a powerful yet easy-to-use music production tool.
To add sound effects and create a musical score for a film, use the Launcher to find GarageBand, then open it. This tutorial is for version 10.1.6, which uses a guitar and amplifier as an icon.
At the “New Project” screen select “Empty Project” then click the [Choose] button.
Select “Software Instrument” then click the [Create] button.
From the Track menu choose “Show Movie Track”
From the File menu choose “Movie” then “Open Movie”
Navigate to your Google Drive’s project folder and select the movie file for which you will be creating music and sound effects.