* In PowerPoint 2003, follow these steps:
1. Go to the first slide, and select Insert -> Movies and Sounds -> Sound from File to insert the sound.
2. After you click OK, you can see a message pop up asking how you want the sound to start in slide shows. Click Automatically.
3. Select Slide Show ->Custom Animation to enter the Custom Animation pane. Your soundtrack is listed below in the pane.
4. In the drop-down list of the sound, choose Effect Options. Then the Play Sound interface appears. In the Effect tab, click the radio button beside Stop Playing After XXX slides to enter the total number of slides.
5. Next, check the settings in the Timing tab. To play automatically, you can set to Start After Previous with a 0-second delay. Click OK to exit the dialog box.
6. Then go to Sound Settings tab, adjust the sound volume as you like. You can also tick to hide sound icon during slideshow if you don't want it appear.
Now you can play the soundtrack from the very beginning to the end of your slideshow.
* In PowerPoint 2007, do as follows:
1. On the slide, click Insert to pick the Sound drop-down list on the very right below the tab. Select Sound from File in the list and choose the specified sound track from your hard drive.
2. Click OK and a message prompts, saying "How do you want the sound to start in the slide show?". Click Automatically.
3. Now click the sound icon to display the Options tab. Click on the tab, and set Play across slides for Play Sound.
At the moment, it is time for you to play the entire slide show while enjoying this pleasant sound score.
Get more details at http://www.dvd-ppt-slideshow.com/powerpoint-knowledge/sound-across-slides.html