I have a couple of movie trailer files that use AAC 5.1 audio. This used to be decoded by libfaad, but is now being decoded by FFmpeg instead ever since this changeset:

changeset:   12289:26c41c4d19b0
user:        Torsten Jager <t.jager@gmx.de>
date:        Thu Feb 21 22:41:17 2013 +0200
files:       src/combined/ffmpeg/xine_audio.list
ffmpeg_audio_decoder: enable mpeg4 audio

Unfortunately, the FFmpeg decoder is broken somehow, and causes xine to freeze after a few seconds playback. Possibly FFmpeg is being configured incorrectly because xine-ui's OSD claims that the audio is "1 Hz" instead of "48000 Hz".