If you need music files, you can download them from here, here or here (the last link is great, but you need to make an account). This is free music, you won't find famous chart songs on here and it is illegal to try to download copyrighted music from YouTube.

If you want sound files, sound effects, you can download them from here.

This is a great resource to get static/animated images from, you need to save the images as files, not copy and paste them.

Use the Microsoft ClipChamp website for video editing and to combine different multimedia resources into videos.

Use the Audacity program installed on the school computers to edit audio files.