I´ve been through the forum up and down, but I´m still missing one answer;)
To have a slideshow with nice back and forward buttons I would consider using a bunch of single menus instead of the slideshow feature. But what happens to audio? Can I somehow manage to play a single audio file underneath all those "slideshow"-menus?
Or is there any other way?
That is not possible with DVD-Video. The DVD-Audio spec allows it, but not DVD-V.
This is not an Encore limitation; it is a limitation of the spec itself.
For an excellent example of how to "simulate" this using audio that is sufficiently repetitive with no large dynamic or tempo changes, please explore the menu system on the "Lemony Snicket's A Series of Unfortunate Events" DVD.