First, find the MP3 audio track you want to use as your background music and rename the file as “useraudio.mp3”. Then open your Photoshop CS2 application folder and navigate to Presets>Web Photo Gallery. Inside that folder look for either Flash Gallery 1 or Flash Gallery 2. Depending on which gallery you chose within Photoshop, drag-and-drop your audio file into the corresponding folder. That’s it! You’ve got background music.
So you rename the MP3 file, drag it into your Gallery folder, THEN have PS build your gallery. You DON'T drag the MP3 into the site folder. It automatically launches (thanks for the option, Adobe) when the page loads.