I managed to work around the gstreamer0.10-plugins-bad-extra problem via the pool repository here http://repository.maemo.org/pool/fremantle-update2/. If you go to free/g/gst-plugins-bad0.10/ for the actual areml.deb file and you could possibly need to go to http://repository.maemo.org/pool/fremantle-update2/free/w/wildmidi/ for the libwildimidi0 armel.deb file also.
Run dpkg -i on any dependencies before doing the same for gstreamer0.10-plugins-bad-extra and then apt-get install decoders-support should work fine ;)

Quand le N900 lit n'importe quel format, on ne peut qu'adorer ça !