Cannot playback MP4 / AAC (.m4a) on (fresh install of) Windows 7

I'm trying to playback MP4/AAC files through QtMulitmediaKit. It works on many platforms, but not on Windows 7. The files can be played without problems using the preinstalled Windows Media Player.

If I install the "K-Lite Codec Pack - Basic [1]" playback through QtMultimediaKit works as expected. However Windows 7 comes with a preinstalled DirectShow filter[2] for MP4/AAC that should make installing another filter unnecessary.

[1] http://www.codecguide.com/download_kl.htm
[2] http://msdn.microsoft.com/en-us/library/dd390676%28v=vs.85%29.aspx