It's doable, but takes a bit of work.
Instead of using Insert > Sound, go to your Controls Tool bar (Turn it on in
View if it isn't on the screen) and in click the more tools icon (botton
right corner of the controls tool bar) and you will be presented with a list
of "controls" you may insert into PowerPoint. Scroll to the end of the list
to Windows Media Player, select it, and then place it on your slide.
Next right click the control (Media player) and select Properties. In the
URL property enter the path and name of your music file. Move up to the top
of the properties box and select custom. There sould be a pull down menu
now available in the custom proerty. Click it. In the dialog box provided
select "Loop"
As media player is an external control, PowerPoint won't reset it when it
loops through your slides. The only down side is that as PPT has no control
over it, you must manually stop it when you are done with the presentation.
Austin Myers
MS PowerPoint MVP Team
Solutions to Multimedia in PowerPoint
www.pfcmedia.com