I want mp3 music to play through the slide presentation.
All the correct procedures are followed:
Click on the slide where you want it to start playing;
In the Side bar, click Slide Transition;
On the Sound dropdown list (under Modify transition), select Other Sound… A dialog box will open.
Browse to the desired file and open it.
The sound will continue playing until another sound file is opened.
Often the sound refuses to play, even though it will play when clicked on the desktop.
Usually it plays immediately. When the file is saved, then re-opened, it will not play any more.