Maker sure you're on the first slide, in Normal View (if you're still in Slide Sorter view, double-click on the first slide). You will be inserting a song onto the first slide...
Go to Student Share - using the shortcut on the Desktop or from the folder on the taskbar...
In a folder, go to Student Share>Koppejan>Music... find a song you'd like to use.
Drag the file from the Student Share folder onto Slide #1.
With the song icon still selected, go to the new Playback toolbar that popped up... click on "Play in Background".
This will make the song play for the entire slideshow, starting automatically and hiding the song icon.
List to your song.
With the song icon selected, either right-click and "Trim Audio" or use the button on the Playback toolbar.
Move the green slider to a good starting part of the song. Do not move the red slider!