We are aware of the VirtualDJ import issue, I've subscribed you to the idea so you'll be updated when there's a fix:

https://support.mixo.dj/t/failed-virtual-dj-database-xml-import

You can import audio files via the Tracks (mp3, WAV etc.) import or the Playlist (m3u) import

https://support.mixo.dj/guide/import-songs-mp3-etc

https://support.mixo.dj/guide/import-playlists-from-virtual-dj-m3u