Follow these steps to loop a music (or any sound) file across slides in PowerPoint 2007:

1. Navigate to the slide where you want the sound to begin playing, which is usually the first slide of the presentation. On the Insert tab of the Ribbon choose Sound | Sound from files. as shown in Figure 2 .
   
   
   Figure 2: Sound from file

1. This opens the Insert Sound dialog box as shown in Figure 3 . Locate the sound file you want to play, and click OK.
   
   
   Figure 3: Insert Sound
2. This will insert the sound in the slide -- Figure 4 shows the inserted sound icon that you'll probably find right in the center of the slide and at the same time PowerPoint brings up a message window asking how you want the sound to start in the slideshow. You can choose the When Clicked option, but Automatically works best -- so go ahead and select Automatically.
   
   
   Figure 4: Play automatically
3. Select the sound icon to activate the Sound Tools Options tab on the Ribbon as shown in Figure 5.
   
   
   Figure 5: Options tab of the Ribbon
4. In the Sound Options group, click the Play Sound drop down list and choose Play across Slides (see Figure 6 ).
   
   
   Figure 6: Play across Slides
5. Select the Loop until Stopped option on the Sound Options group as shown in Figure 7 .
   
   
   Figure 7: Loop until Stopped
6. In all versions of PowerPoint, you might see a sound icon on the slide where you inserted the sound - if you don't like the icon on the slide, just drag it to anywhere off the slide - your sound shall still play fine.

Courtesy of Geetesh Bajaj, http://www.indezine.com