PowerPoint 2007: Add sound to slides

To add transition sound:

Select the slide.
On the ribbon, click Animations > Transition Speed and select the sound you need.

To play background music across a whole PowerPoint presentation:

Select the first slide.
On the ribbon, click Animations > Transition Sound to get the pop-up window.
Scroll down to the very bottom of the form until you see Other Sound and select it.
The "Add Sound" pop-up window should now appear. Navigate to the Background Music Loop that you wish to use on your hard drive and select the file. The sound file needs to be .wav form.
To loop the Background Music Loop, select the tick box Loop until next sound.