Hi Sasha,

The problem is that PowerPoint does not play the audio, it simply hands it
off to the operating system to play.  Therefore there is no sync or sync
points to any specific slide, except the one that starts it.  If there is a
place where you might want to start the music, other than the first one, you
will need to create a new sound file that has been trimmed to start where
the other one would be and start it manually when beginning at this slide.

I should also toss out this warning, because my crystal ball tells me that
you are trying to create a tight sync'd "music video"-like presentation.
Sound/slide sync does not hold between machines or even runs within the same
machine.  You would be far better off if creating this type of presentation
using something like movie maker (probably already on your computer) or a
third party video editor.