I realize I might be a little too late with a solution, but I had this problem and eventually figured it out after a week. Hopefully it will help you and anyone else looking for a solution. Here it is:

First, you need to make sure you have Qt with phonon support. I believe the 'standard' version of Qt does not have phonon support, so you might need to download the Qt source and build it with phonon support.

You need to run "macdepoyqt" in the base directory of your application. (Macdeployqt should either come with qt or can be built.) More specifically, go to the directory where "MyApplication.app" is located, and call "macdeployqt MyApplication.app". This will copy all the necessary Qt frameworks and dylibs into your app folder. This is very important for phonon because the phonon backend needs lots of files and codecs. (Simply copying the phonon.framework and dylibs yourself will not work.)