Daniele, I admit that the StackSlider is not the proper tool for what you try to achieve. I am very sorry that I mislead you. I mentioned it only as an example of what is available for use in tandem with the Stacks plugin, not as the thing to be used for your particular purpose.
As I am now more familiar with your situation, I want to repeat: zeebe’s was the best and possibly the only suggestion you should consider.
Use iMovie (or a similar app) to create your slideshow. You will be able to add a soundtrack that will play, pause and re-start in-synch with images. You will also have a great selection of transitions between images. You can even set the automatic timing of each image synchronized with the length of your soundtrack, or you can fade-out the audio if the track is too long.
Then, after researching possibilities, choose one of the plethora of video stacks for including your video-slideshow in your website. You may want to consider a stack that lets you use a video that is “warehoused” either on your own host’s server, or on servers of YouTube, Vimeo or others – to make your RW project lightweight and performing better.
P.S. As you could see, you should always try first products that let you download and use a trial version, so you can make sure that the product is right for you…