Talk about synchronicity...
Last week's big debate was (again) whether Phonon is a Good Thing or not. And coincidentally, one of my tasks last week was to implement sound notifications in the cross-platform Qt4-based chat application we're building at Hyves. Qt4, so the first thing I reached for was Phonon.
I had looked at phonon before, but that was in a failed attempt to make a phonon video flake shape for koffice. Failed, because I couldn't find a good way to make phonon output video data something that isn't a real widget. That may well have been solved now with widgets-on-canvas for something like QGraphicsView, but whether that solution is portable to flake, I'm not sure yet.
But last week was the first time I tried to use the Phonon api myself: and really, it is a wonderfully easy to understand api. I didn't have to do much coding at all. It was more of a problem to actually get Qt compiled with phonon on all three platforms, and the biggest problem was that the macdeployqt tool bundled with Qt doesn't deploy the plugins, only the library frameworks, although the documentation suggests it does. That took quite a bit of time to figure out.
But all in all, I think that Aaron is right: phonon is one more good reason to choose Qt for your cross-platform application.