\version "2.18.2"

\language "english"

\score {
  \new Staff {

    \time 4/4
    \clef treble

    \relative c''
    {
      c1
      \repeat volta 2 {
        c d e f
      }
      \alternative {
        {
          g f e d
        }
        {
         % \set Score.voltaSpannerDuration = #(ly:make-moment 4/4)
          \set Score.voltaSpannerDuration = #(ly:make-moment 4/4)
          \repeat unfold 2 {
            \repeat unfold 12  {g8 a} g f e d c d e f
          }
        }
      }
      b1 a g f e
    }
  }
}