I would suggest u take an hour and try running show from FPP. It will take more than that to try and figure how to slave a FPP to xlights. No one has done that before. Here is quick steps
Do not use MP3 for your sound file in xlights, use MP4. Backup everything, "file,backup" in xlights.
Now create a new sequence from sound file, click that u have an xml sequence file
For the sound file, give your MP4 movie.
Give your current sequence xml
Press save.
Now you have a sequence that uses the sound file from the movie. Audacity, btw, can open MP4 files.
Convert to fseq, send to pi player. Send MP4 movie file to pi player. U no longer need the MP3 sound file.
Playlist , u pick fseq file and MP4.
When it plays, movie is launched and your lights flash