Go to the slide where you want to begin your audio track (defaulted as the first slide), and select Insert -> Movies and Sounds -> Sound from File to insert the sound.
After you click OK, you can see a message pop up asking how you want the sound to start in slide shows. Click Automatically.
Select Slide Show ->Custom Animation to enter the Custom Animation pane. Your soundtrack is listed below in the pane.
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 (which indicates how many slides you want this song to play across) to enter the number of slides you intend to add. For example, you can enter a number from 1 to 999 for this field. Then the sound will play throughout the specified slides.
