Tnx John. 1/2 way there.
Here's what I have.
I have 25 slides in complete presentation.
Slide 1 is a segue (5-6 photos) that fade in and out and loop until the main presentation is started (slides 2-24) by action button to advance to CSS #2 which is slides 2-24 advancing by mouse clicks and then ending with slide 25 with 5-6 photos again fading in and out like 1st slide.


IOW - I had 3 Custom shows with action buttons advancing to next CSS.
I created a new #1 slide according to instruction above, inserted music but couldn't get it to play so I created a new CSS #1 (renaming the other CSS 2,3,4) with action button advancing to CSS 2 and music plays, etc. EXCEPT when I click action button in CSS 2 to proceed to main presentation(CSS 3) the music does not stop...