Pithos has undeclared dependencies on py27-pylast and one or more of the plugins in gstreamer-plugins-all.

Upon initially installing and running pithos, you get a Python stack trace about failing to import pylast.  Installing "pkg install py27-pylast" fixes this.

After installing pylast, running pithos produces the following errors, with the GUI giving a similar message about GStreamer missing a plugin:

gdwatson@dweorh:~$ pithos
** Message: pygobject_register_sinkfunc is deprecated (GstObject)
ERROR:root:Could not bind media keys
ERROR:root:Gstreamer error: Your GStreamer installation is missing a plug-in., gsturidecodebin.c(1285): GstElement *gen_source_element(GstURIDecodeBin *) (): /GstPlayBin2:player/GstURIDecodeBin:uridecodebin0, 12
ERROR:root:Gstreamer error: Your GStreamer installation is missing a plug-in., gsturidecodebin.c(1285): GstElement *gen_source_element(GstURIDecodeBin *) (): /GstPlayBin2:player/GstURIDecodeBin:uridecodebin1, 12
[More omitted.  It goes on for a while.]

The Pithos web site says that the necessary codecs are usually packaged as either "good" or "bad" GStreamer plugins.  Installing gstreamer-plugins-good and gstreamer-plugins-bad does not fix the issue.  Installing gstreamer-plugins-all does, so the missing plugin is in there somewhere.

Fix: 

Declare a dependency on py27-pylast and the mysterious codec.
How-To-Repeat: "pkg install pithos" on a system without py27-pylast or gstreamer-plugins-*; run pithos and watch it die.  "pkg install py27-pylast"; run pithos and see the decoder errors.