I think you will have to do what Scott suggested and embed the sound in the powerpoint presentation.
What is probably happening is this...
- You create your PowerPoint presentation and say you want to include to sound file "C:\My Music\Intro.mp3".
- When the presentation is ran on your PC, it will see that it needs to look for a sound file at "C:\My Music\Intro.mp3". It'll find this and play it.
- You'll then send this file over to someone via email.
- They will try to run the presentation. At this point powerpoint thinks, "I need to play the sound file at C:\My Music\Intro.mp3". It'll look in this directory but of course this file won't be here (the file is on your PC in this location, not theirs). Due to this, the sound won't play.
So just to summerise - embed the sound. Otherwise you will have to make sure that the person viewing the presentation has the sound files in the same place on their PC as where you have them.
Quick side note, sounds and transitions in powerpoint presentations often annoy a lot of people. It is also distracting if you are trying to talk over this presentation while this is going on. So if the sound isn't an important part of the presentation, might be best just to leave it out.