Really a digital book represents a potentially infinite canvas to lay the images out upon. The screen can be a page, but it can also be a window. What happens if you open that window? Or if you break it? You could even just publish beyond a standard screen resolution, provide the highest resolution images possible and let the viewer decide how to experience your images. Zoom in. Zoom out, see details, take it all in.
Audio is a beguiling option, but perhaps it breaks the flow too much — causes an interruption that breaks the slow media aspect of absorbing all the senses that a photograph communications, through the eyes alone. Don't try to lay out your SoFoBoMo as a book, that is then poorly displayed on a screen. Instead, consider using the PDF to lay the images out optimally for an on screen display. Break the two page, odd, even print metaphor. Flow the images backwards. Use a diagonal arrangement. Up and down. Reverse direction along the way. Take a sharp right turn at an appropriate point in the story. Diagonals. Zig Zag across the canvas. Wavy lines. Choose what suits your images, let the form follow the function or the flow.
Physical books are great too — they have presence, weight, gravitas that a PDF can't reach. But it is so much simpler to get a PDF in front of a huge number of people around the world, quickly. The resolution is much higher on a page than a typical screen (5 or 6x as much) but that resolution is fixed — on a screen the resolution can be dynamic. Zooming is a feature, that can be used. Books have a huge bandwidth, but the latency is pretty bad. Digital books are the opposite. The physical size of the page is a hard limit with a particular book format. Not so for a PDF. Same way the flow is much more dictated by a real book. There is a lot of potential to design a PDF for SoFoBoMo that is a first class representation of your work, rather than just a stop-gap or a shadow of the real printed book you are going to make. It might be a horrible design decision, but then exploring new ideas is part of what SoFoBoMo is all about.