Another self-reply which may help anyone else trying to use Gstreamer apps in Easy Debian.

Using the gst-launch-0.10 from gstreamer-tools I can play .wav files from the command line no problem, but if I play .ogg files (including those supplied with Tuxpaint, which play perfectly from within that program), all I hear is a faint, crackly sound, even with all the volume settings in gnome-alsamixer set to full. It's like listening to SW radio from another continent with the volume down low. The files do play, but badly and quietly.

My solution... erm. Dunno. I have the gstreamer-base-plugins installed, which claims to include OGG support, and the files do actually play, but not properly.

My work-around is probably going to be to convert the GCompris OGG files to WAV format (maybe leaving them with the same filenames so I don't have to edit all of GCompris' python scripts) and hopefully that'll do the trick. I'm not likely to use any other Gstreamer apps so this is a one-off thing.

If anyone knows of a more elegant solution, please let me know.