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.
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:
Can I use MIDI files with Rhythmbox?
Rhythmbox uses the GStreamer media framework for actual playback and a lot of other functionality, so in general Rhythmbox plays exactly those formats that are supported by GStreamer. GStreamer, on the other hand, uses a plugin system where each format is supported by a plugin. Thus, the formats supported by GStreamer (and Rhythmbox) depends on which plugins you have installed. Different distributions may ship with a different set of plugins by default.
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