Hi Ludo and Ricardo,
The 'uridecodebin' error is a result of GST_PLUGIN_SYSTEM_PATH not being set. However, the recent patch to gstreamer adds that to its native search paths, so that error should be resolved now on an updated system. I had a line defining GST_PLUGIN_SYSTEM_PATH in my .bashrc, but I was able to remove it after that patch was applied. Rhythmbox is working without errors for me.
I've also realized that some of the packages I put under inputs should probably be propagated inputs, so I'm attaching an updated patch which changes that.
I am currently using guix over ubuntu.
Thanks,
David