Mathilde's dashboard

I had the same problem too. I had Songbird installed with the Quicktime add-on but it wouldn't play AAC, M4A files, even though I had Itunes with Quicktime installed as well (because I need the Itunes Store).
Since there was no way to solve this problem, I uninstalled Apple Quicktime and replaced with Quicktime Alternative. Magically, Songbird started playing AAC and M4A files! And Itunes works too, so everything seems to be perfect now.