For office 2007:

1. Click the drop down arrow of Sound and select Sound from File…. A window will appear allowing you to browse to music file that you want to associate with the slide. After choosing the right music, click OK.
2. You will be prompted to choose the way the music would play. Click Automatically to set it play automatically when you go to the slide. Click When Clicked to set it play only when you click the sound icon.
3. After this, an icon of a speaker will appear in the middle of your slide. You are allowed to move the icon to any location you desired on the slide.
4. Adding process completed. The music would only play for the single slide.

If you have PowerPoint 2003, the difference is that the operation directory is main menu Insert > Movies and Sounds > Sound from File…. The other sub-steps are nearly the same.


Then you can Set the mp3 effect:
more options refer to How to Add Music Song to PowerPoint Presentation