Once you upload your own video, its original audio is added automatically (if there is such).
In case you decide to have only music from Promo library, you can mute your uploaded video. Each clip's sound is muted separately.

Note. In the editor, you will be able to hear both sound of your uploaded video and music from Promo library that will be added automatically.

If you would like to keep the uploaded video sound and remove Promo's music, you can select “No Music” option in Music tab. 

Hover over 'Silence' track that will appear, and hit 'Use Track' button. Now you have only your own uploaded music content playing :)