An idea should be to follow a tutorial from Qt official site, then import an example and use it, look the code, learn from there.
After migrate to Qt5, and after follow the tutorial for read video, after look at the example, you should be able to construct a little application for just read video with QMultimedia. (don't forget to incule multimdeiawidgets in the pro file also...).
there is also some example on youtube you can trust and follow for that.
After, do more than just read/record a simple video will ask more implication and maybe will be limited with just Qt without plugins.
But just for read video, you doesn't need phonon at all. Do it simple and easy first.