I believe that libxine1-ffmpeg is required for AAC in xine. Although that package was available with the previous 1.1.16.3 xine-lib, it unfortunately appears that package is not included anymore for lmce since the update from 1.1.16.3 to the current 1.1.16.4. If you attempt to install libxine1-ffmpeg apt will downgrade your current xine-lib and you'll lose (at the least) the current updated .mkv support.
J.