In Vixen, did you go to Sequence > Audio, then click the Remove Audio button? If you just deleted (or moved) the MP3 file, Vixen will play the sequence without audio, but leave the mp3 file name in the sequence file - this will confuse xLights.
To fix, go to Sequence > Audio, uncheck "Resize the sequence to the audio length", then assign an audio file to the sequence (audio file should be shorter than the length of your sequence). Now go back to Sequence > Audio, uncheck "Resize the sequence to the audio length", and click the Remove Audio button. Save your sequence. It should now play in xLights.
Good Luck!
Matt