I had the same problem for a frustratingly long time...I have the answer.
First of all you need to deal with the music...it has to be embedded into the
slideshow. Go here and read how to do that in this tutorial...
http://www.technologytrish.co.uk/pptemailmusic.html
That way the music will, in fact, become a part of Powerpoint so it won't
have to take the time to "find" the file. After you have that down and you
have rehearsed the timings, view your show, and if the timings are a bit off,
you can then go back to each slide and fine tune the amount of time you want
each slide to progress to the next. Sometimes it takes a few times of
watching the show and adjusting the times but, I am glad to say, it is
possible. Go to Slide Transitions, Advance Slide, and underneath
"Automatically after", click the arrow up or down to adjust. A tip...if you
are adding custom animation, you need to make sure that the time each
animation takes, does not exceed the length of the slide otherwise you will
have timing problems. In other words, if you choose a custom animation for a
horse gif to walk across your slide, for instance, make sure that it has
finished walking in a bit less time than it takes your slide to progress.
Fix those first before you adjust your timings and you'll save time. If you
need more help, you may email me at (e-mail address removed). Hope this
helps.