This took way too much futzing around to get working (or perhaps I’m just too used to Mac OS X now), so here’s a no-frills guide to get it working:

• Add the lines
  deb http://marillat.free.fr/ unstable main
  and
  deb http://pessoal.onda.com.br/rjamorim/debian/ ./
  to your /etc/apt/sources.list file.
• apt-get update; apt-get install libfaad2-dev
• apt-get -t unstable install libstdc++5
• If you’re on x86, download the i386 .deb of my patched version of Frederic Fondriest’s xmms-mp4plugin XMMS input plugin.
• If you’re exotic enough to not have an x86, download the debian source, and run
  fakeroot debian/rules binary
  to build the .deb for you. You need the g++-3.3 and automake1.7 packages installed to compile this thing.
• Execute sudo dpkg -i xmms-mp4plugin_0.2-1.andrep_*.deb to install the package.
• Run XMMS and enjoy.

Useful links:

• http://rarewares.hydrogenaudio.org/debian.html: RareWares Debian repository for faad2, faac, aac/mp4, lame, lame-ha, lamip, cue2toc, shntool
• http://www.xmms.org/comments.php?show=P211: Original sources for Frederic Fondriest’s MP4 XMMS input plugin

(Random keywords to be more search-engine friendly: Debian, Linux, XMMS, iTunes, MPEG-4, MP4, AAC, M4A.)