\version "2.18.2"

% a list of needed var names as symbol-list
AllMusicNames = #'(DescantMusic SopranoMusic PianoLHMusic)

% define AllMusic as music function - to be called with \AllMusic
AllMusic = #(define-music-function
             (parser location)()
             (make-music
              'SimultaneousMusic
              'elements
              (map
               (lambda (n)
                 (let ((mus (ly:parser-lookup parser n))) ; get the object with the name n or an empty list
                   (if (ly:music? mus) ; if mus is music, return it, otherwise return void-music
                       mus
                       (make-music 'SequentialMusic 'void #t))
                   )) AllMusicNames))
             )

DescantMusic = \relative c'' { ees4 e f fis }
SopranoMusic = \relative c'' { bes4 a c b }

\AllMusic