MIDI file creation - or not?

[Ben Potter]

I am very new to Lilypond but hope to use it to develop my composition 
skills, initially by writing Anglican chants. I have no keyboard skills 
and, at 71, am too old to learn them. I hope, therefore, to use Lilypond 
to create MIDI files to play so I can hear the results of my efforts.

I took the example chant and engraved it as a test - fine. I then added 
a \midi{} command in, I believe, the correct place but still get no MIDI 
file to play. I am stuck - please can someone help this beginner?

I append the file I've been using.

Many thanks,

Ben

--

We have been broadcasting our presence to the Universe for 100 years now. If 
there is a detachment of Galactic
Pest Control within 100 light years, they are already on the way.
        -- Anon.

\version "2.18.2"
#(ly:set-option 'midi-extension "mid")

SopranoMusic = \relative {
  g'1 | c2 b | a1 | \bar "||"
  a1 | d2 c | c b | c1 | \bar "||"
}

AltoMusic = \relative {
  e'1 | g2 g | f1 |
  f1 | f2 e | d d | e1 |
}

TenorMusic = \relative {
  c'1 | c2 c | c1 |
  d1 | g,2 g | g g | g1 |
}

BassMusic =  \relative {
  c1 | e2 e | f1 |
  d1 | b2 c | g' g | c,1 |
}

global = {
  \time 2/2
}

#(ly:set-option 'midi-extension "mid")

% Use markup to center the chant on the page
\markup {
  \fill-line {
    \score {  % centered
      <<
        \new ChoirStaff <<
          \new Staff <<
            \global
            \clef "treble"
            \new Voice = "Soprano" <<
              \voiceOne
              \SopranoMusic
            >>
            \new Voice = "Alto" <<
              \voiceTwo
              \AltoMusic
            >>
          >>
          \new Staff <<
            \clef "bass"
            \global
            \new Voice = "Tenor" <<
              \voiceOne
              \TenorMusic
            >>
            \new Voice = "Bass" <<
              \voiceTwo
              \BassMusic
            >>
          >>
        >>
      >>
      \layout {
        \context {
          \Score
          \override SpacingSpanner.base-shortest-duration = #(ly:make-moment 
1/2)
        }
        \context {
          \Staff
          \remove "Time_signature_engraver"
        }
      }
     \midi { }
    }  % End score
  }
}  % End markup