Go to the TRANSITIONS tab.
With one slide selected, go to the drop-down menu and apply a transition to the slide. Add a variety of transition types to your slides.
Go to Slide Sorter view using the VIEW tab ...
...or the button at the bottom right:
Zoom out so you can see all your slides at once.
Then go back to the TRANSITIONS tab and select all your slides (CTRL+A).
On the right side of the toolbar, uncheck the box for "On Mouse Click".
Now check the box for "After", and set the Advance Slide amount of time for all your slides to 20 seconds.
CTRL+click on the song slides to select them, and reduce the amount time to 10 sec.
Double-click on the first slide to go back to Normal view. Reduce the amount of time to 3 sec.
Go to the SLIDE SHOW tab and Set Up Slide Show.
Then set Show Type to "Browsed by individual" (this will make slides advance using the timings you've set up).
Test it out. Hit F5 and play your slide show. It should be self-running, with the songs playing for 2 slides only.