put the mp3 file and your presentation in the same folder, just
insert>>sound then insert your mp3 file into your presentation, when
send to other computer you should send the whole foler rather than the
single presentation http://www.ppt-to-dvd.com/blog
I often use Wondershare PPT2Flash Standard to insert song files with
one click. It's free to use this feature. And time-saving.
You can download the free trial one to help you, don't need to learn
the complicated steps. http://www.sameshow.com/download/ppt2flash-download.html