Follow these steps to loop a music (or any sound) file across slides in PowerPoint 2004 on the Mac:

1. Navigate to the slide where you want the sound to begin playing, which is usually the first slide of the presentation. Go to Insert | Movies and Sounds | Sound from File (highlighted in red), as shown in Figure 2 .
   
   <img src="images/tutorial28.jpg">
   Figure 2: Sound from file
2. Locate the sound file you want to play, and click Insert, as shown in Figure 3 .
   
   
   Figure 3: Insert Sound
3. PowerPoint will prompt you if you want to have the sound on the slide play automatically, as shown in Figure 4 . Choose Yes.
   
   
   Figure 4: Play automatically
4. Control-click (or right-click) the sound icon placed in the center of the slide, and choose Custom Animation from the contextual menu as shown in Figure 5 .
   
   
   Figure 5: Right-click menu
5. This opens the Custom Animation dialog box that you can see in Figure 6 -- with the inserted sound pre-selected as the object to animate.
   
   
   Figure 6: Custom Animation
6. With the sound object still selected, click the Effect Options button to bring up a dialog box of the same name. Select the Media Options tab as shown in Figure 7 -- and type in "999" in the Stop playing After text box (see Figure 7 ).
   
   Note: You type 999 here since that's the largest number that PowerPoint will allow you to add here.
   
   
   Figure 7: Effect Options
7. Click OK to get back to the Custom Animation dialog box -- and click OK again to get back to your slide. Now control-click (or right-click) the sound icon placed in the center of the slide again, and choose the Edit Sound Object option (refer to Figure 5 above).
8. This summons the Sound Options dialog box that you can se in Figure 8 below. Make sure you check the option that says Loop until stopped (highlighted in red).
   
   
   Figure 8: Sound Options
9. Click OK -- 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