Hello guys! So I've been looking at this post since half an hour ago, having the same problem. But instead of all that complicated whatever stuff you guys had to do with the build in and build out (I couldn't understand at all, my head is still hurting from reading the whole process when the original person who asked the question posted a solution), I've found another. Anyways, I went on Youtube and short story short, here's the link to exactly what you guys have been asking for:
https://www.youtube.com/watch?v=vmk952MLtC4
An easy, peasy, simple solution to your problems! However, when you use this, the moment you press 'play presentation' the music or soundtrack audio will start immediately. The only way to stop it then is to exit out, so just in case this wasn't what you wanted, please don't reply with angry or upset comments. Thanks! Have a nice day. (:
(P.S. oh and of course, credit to whoever made this video! It really saved me.. and my grade so thanks!)