Follow these steps to loop a music (or any sound) file across slides in PowerPoint 97 and 2000:

1. Navigate the first slide of the presentation and choose Insert | Movies And Sounds | Sound From File… as shown in Figure 2 .
   
   
   Figure 2: Sound from file
2. This opens Insert Sound dialog box as shown in Figure 3 . Navigate to the sound track you want to insert and click OK.
   
   Note: PowerPoint allows you to insert several types of sounds in various formats -- but WAV, WMA, and MP3 work best.
   
   
   Figure 3: Insert Sound
3. 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 do you want the sound to play automatically in the slideshow. Choose Yes.
   
   
   Figure 4: Play automatically
4. Right click the sound icon and choose Custom Animation (see Figure 5 ).
   
   
   Figure 5: Right-click menu
5. This will open the Custom Animation dialog box as shown in Figure 6 , in the Multimedia Settings, Check the " Play using animation order " option and under While playing choose the Continue slide show radio button. For the Stop playing option, choose After and type 999 (refer to figure 6 ).
   
   
   Figure 6: Multimedia Settings
   
   Note: You type 999 here since that's the largest number that PowerPoint will allow you to add here.
   
   When done, click OK to get back to your slide.
6. Right click the sound icon again and choose the Edit Sound Object as shown in Figure 7.
   
   
   Figure 7: Edit Sound Object
7. This brings up the Sound Options dialog box (see Figure 8 ).
   
   
   Figure 8: Sound Options
   
   Under Play options check the Loop until stopped box (highlighted in red) and click OK.
8. 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