Music in NextGen Gallery

Not sure if I’m missing something… I put a full URL in the slideshow options field for background music, but nothing plays. Tried a WAV file, then a WMA file, but get no sound. (I’m still trying to convince my client NOT to use it.)

If I go directly to the URL the music plays, so I’m at a loss. I couldn’t find any documentation on music types or limitations for this feature.