The process is the same as inserting a song to be played on a slide, only then customize The performance of this song to sound on every slide
The first is to insert the first slide, a sound object. This item is in the Insert tab . We asked to state the sound file you want to use.
Once inserted the sound object, we need to go to the Tools tab Sound (this tab does not appear if the sound object, with a speaker, not selected)
In this tab, audio tools, we have the Sound Options section where this one down with several options: automatically, over and Click Play across slides . This last option is that we have to select.
From this moment, when we put in presentation mode, start playing the selected song until the end of the presentation (if we check the box of the same paragraph, Loop until its interruption)
I have not found this to PowerPoint 2003, but is the same in the PowerPoint version 2007 and PowerPoint 2010
the 
