Fotis Pastrakis

Was it a free demo? If it’s free, don’t expect that you can do changes.
They all use their own CSS with the demo content combined.

Do you have a backup of your website (wordpress files and database)?
If yes, upload it through ftp and phpmyadmin.

A plugin cannot do this work because it’s up to YouTube’s credentials. A single youtube account is not an open account which anyone can upload a video, but it’s a private one. Although, you can make your videos and ask from students a video response with their own accounts.

If you got the resources you can create a YouTube like service for this job but I wouldn’t recommend that.

Have you set your own music through theme or do you have something embedded? if you haven’t did something of those two, you have to remove the sound from your theme which you can find inside the customize of the theme.

Hello, you can set them as downloadable buttons if your theme supports it. Alternatively, you can search for a suitable plugin for this function.