Hey - I'm new here, but I'm actually working on a completely free graph/node-based tool for just this purpose (Right now it's Windows only). My daughters are both excited about the idea and I'm a programmer by trade, so....
Anyway, the tool just generates script that runs in Torque 3D/2D - so an existing game engine that is actually meant for video games, but will handle general image presentation nicely along with having all of the sound tools you could need. But it is extensible - you could write a plug-in for it that would export script for any other engine that uses scripts, really. Right now I'm working on the last bits of the project management tools (folder browsers, script management, project organization) before I start on the visual novel plug-in. The source for the whole project is available under MIT license here : https://github.com/RichardRanft/T3DConvoEdit (originally it was going to be a simple conversation tree editor, but I extended the plug-in system to make it more flexible). Feel free to use it as a jump-off point if you have programmers on your team.
The only reason I dropped this here is that I felt it would fit in as "tools" - hopefully someone gets some use from it.