I had the same problem in Lucid Lynx. Rhythmbox 0.12.8 would not show duration "Unknown" and disable the slider bar.
However, installing
gstreamer0.10-plugins-ugly
fixed the problem.
I confirmed this by uninstalling gstreamer0.10-plugins-ugly, which brought back the behavior, and installing it again, which fixed it.
I had the same problem in Lucid Lynx. Rhythmbox 0.12.8 would not show duration "Unknown" and disable the slider bar.
However, installing
gstreamer0. 10-plugins- ugly
fixed the problem.
I confirmed this by uninstalling gstreamer0. 10-plugins- ugly, which brought back the behavior, and installing it again, which fixed it.