Some news on this: I installed the gnome-core package using aptitude today. Among 300 other dependencies, it installed a bunch of gstreamer packages:
[INSTALL, DEPENDENCIES] gstreamer0.10-gconf:amd64 [INSTALL, DEPENDENCIES] gstreamer0.10-nice:amd64 [INSTALL, DEPENDENCIES] gstreamer0.10-plugins-bad:amd64 [INSTALL, DEPENDENCIES] gstreamer0.10-plugins-good:amd64 [INSTALL, DEPENDENCIES] gstreamer1.0-clutter:amd64 [INSTALL, DEPENDENCIES] gstreamer1.0-nice:amd64 [INSTALL, DEPENDENCIES] gstreamer1.0-plugins-bad:amd64 Since then, I do not get error messages anymore when opening a subtitle file. The video doesn't display (that area of the gnome-subtitle UI remains black), but the audio plays fine which makes the software more usable.
