Follow these steps to loop a music (or any sound) file across slides in PowerPoint X 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, as shown in Figure 2 .
   
   
   Figure 2: Sound from file
2. This brings up the Insert Sound dialog box that you can see in Figure 3 . Locate the sound file you want to play, and click Insert.
   
   
   Figure 3: Insert Sound
3. When prompted 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. The Custom Animation dialog box will open with the Options tab active, as shown in Figure 6 . Also, the inserted sound will be pre-selected as the object to animate.
   
   
   Figure 6: Custom Animation
6. Check the Play using Animation Order option, and choose the Continue slide show radio button -- also change the number in the Stop playing After text box to 999 slides. Check the box to Hide when not playing if you do not want the sound icon to appear when you show the presentation. You can toggle the option to Loop until stopped if you wish for the sound to repeat if the sound is finished yet the presentation is not (refer above to Figure 6 ).
7. You can now adjust timing options for the sound. In the Custom Animation dialog box, select the Order and Timing tab. If you wish for the sound to play right when you open the slide, choose the Media entry under Animation Order that matches the one selected in the left pane, and press the Up arrow next to the list until the selected entry is on top. This ensures that the sound animates (plays) before any other objects do. Of course if you have no other animated objects in this particular slide, then the Up and Down arrow buttons will be grayed out, as you can see in Figure 7 .
   
   
   Figure 7: Custom Animation
8. Also, under Start Animation , choose Automatically , 0 seconds after previous event . This ensures that the sound begins playing as soon as you open the slide, or as soon as the previous object has finished animating if the Media object is not first in the Animation Order list (refer to Figure 7 above).
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