BpmDj comes with two demo tracks, as to explain novel users the basics of the app. Occasionally we swap out those tracks which might leave some users, who used these demotracks in their mixes with gaps.
If you have tracks that rely on some of the past demotracks and you would like to restore those mixes, simply download the relevant demo pack, unzip it on your device and extend your library with those songs.
Yixdee demotracks