Rhythmbox 2.96 doesn't run any midi file on Ubuntu 12.04, even after installing GStreamer and Ubuntu Restricted Extras.
Rhythmbox-Message: Missing plugin: gstreamer|0.10|rhythmbox-metadata|audio/midi decoder|decoder-audio/midi
[0xb5504e40] main decoder error: no suitable decoder module for fourcc `MIDI'. VLC probably does not support this sound or video format.
You can play midi with VLC. Just install
vlc-plugin-fluidsynth
Usually audio-players do not support playing midi file, that strictly speaking are not audio files.
You can play midi file through a MIDI software synthesizer, like Timidity or Fluidsynth.
In the Rhythmbox FAQ you find the following question:
so if you still want to use Rhythmbox for midi files, take a look at the mentioned Wildmidi project.
Install Gstreammer 0.10
Open your Terminal, copy and paste:
sudo apt-get install gstreamer0.10-ffmpeg gstreamer0.10-fluendo-mp3 gstreamer0.10-gnonlin gstreamer0.10-sdl gstreamer0.10-plugins-bad-multiverse gstreamer0.10-schroedinger gstreamer0.10-plugins-ugly totem-plugins-extra
sudo apt-get install gstreamer-dbus-media-service gstreamer-tools ubuntu-restricted-extras
that should do it