Please double check that the file is located on your device and saved in one of the supported formats.
For a full list of supported formats, please follow this link.
Please be aware that the files stored on a cloud or obtained by means of any subscription, e.g. Apple Music, even the downloaded tracks, are not visible in Movavi Clips in order to avoid any copyright issues.
The audios can be added one after another.
If required, you can trim the files.