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If you want the song to start when you start your presentation, open the first slide. If you want it to start at a specific point in the presentation, open the slide you want it to start on. You'll be able to browse for either WAV or MP3 files. In Office 2003, click the Insert menu, select "Movies and Sounds", and then select "Sound from File". Since PowerPoint 2003 and 2007 cannot embed MP3 files, you'll have the most success if you create a new folder on your computer and put the presentation file along with the audio file in the same spot. You can embed WAV files, but this can create a very large presentation file. It is recommended that you use a linked MP3 file instead. In the "Sound" tab, you can select either "Automatically" or "When Clicked" from the "Play Sound" menu. If you set the song to play automatically, check the "Hide During Show" box to hide the button for the audio file. Normally, the song will stop playing as soon as you move on to the next slide. By creating a custom animation, you can force the music to continue playing longer. Set this to the number of slides in your presentation to have the music play in the background the entire time. Click "OK" after you're finished. Since the presentation will not have the music file embedded, you will need to "pack" the presentation and the audio together using the "Package for CD". This will allow you to easily share the presentation with others. You won't need to actually burn it to CD.> Click the Office button, select "Publish", then "Package for CD". Enter the name of the folder you want to create in the "Name the CD" box. Click "Options" and ensure that "Include linked files" is checked. Click the "Copy to Folder" button. A new folder will be created with your presentation and the audio file, along with a PowerPoint player so that anyone can view the presentation, even if they don't have Office.

Summary:
Open the slide you want to start the song on. Click the Insert tab, click the "Sounds" button, and then "Sound from File". Decide how you want the sound to start playing. Right-click on the new audio object and select "Custom Animation". Click the "Multimedia Settings" tab and select the "Continue slide show" option. Select the "After" option and then set how many slides you want the music to continue playing for. Package the file.