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 only music from Promo library that will be added automatically or your uploaded music track. In order to hear the sound from your uploaded video, please click on "Save and Preview", and once the video is saved, you will hear it.
If you would like to keep the uploaded video sound and remove Promo's music, you can select "No Music" option in Music tab.
Hit "Use" on silent track, that shows up, once "No Music" genre is selected. Now you have only your own uploaded music content playing :)