I HAD titles and still pictures loaded on top of a audio clip and it worked fine, but now SOMEHOW the audio still plays and the bar moves but the viewer is just black

I've read and re-read your post and have no idea what you are asking about. Did you create a slideshow? If yes, what application did you use to create it? Did it work previously? Did you check to see if updates are available for the application? What specific version of macOS is your iMac running (10.14.???). What application are you using to play the 'slideshow'? Is is a different application than the one you used to create it? Have you tried restarting your iMac to see if that helps?

It's really difficult to provide useful suggestions without having at least some information about the issue.