Once you upload your own video, its original audio is added automatically (it 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.
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 :)