I never heard of a .xspf file, but a quick search took me to this site.
https://www.lifewire.com/xspf-file-2622613
Here, they indicate that .xspf is not an actual audio file. It is instead a text file that functions as a playlist, pointing to the actual audio files.
So, I made one using the example shown in the article, and it played fine in VLC.
So, if they are actual .xspf files, they should be small and can be opened in notepad. If they're large, they're probably actual audio files that should have a different extension on them.
I'm thinking that the .xspf file is pointing to audio files that have been moved? If the .xspf file is a little dinky file, open it up in notepad and see if it points to actual audio files that are in the location being pointed to.