The German soldiers too had a lot of songs; if they turned up the US
I don't know...  Even in the Welte roll repertoire is a roll with
German war songs, released in the US before 1916:

Roll Nr. 3630, played by Heinrich Burckhard

  "What The German Soldier Sings" : Die Wacht am Rhein; Muss i denn,
  muss i denn zum Staedtele hinaus; In einem kuehlen Grunde;
  Wer will unter die Soldaten; Morgenrot; Ich hatt einen Kameraden;
  Gebet vor der Schlacht; Steh ich in finstrer Mitternacht;
  Oh Strassburg, Oh Strassburg, du wunderschoene Stadt;
  Deutschland, Deutschland, ueber alles.

 [ Later catalogs (1916-1917) changed the title to "Dear Old Germany",
 [ and after the USA entered WW1 the New York artist changed his name
 [ to Henry Burkard! -- Robbie

But I can provide you with literature (in German of course):

  Olt, Reinhard - Krieg und Sprache : Untersuchungen zu deutschen
    Soldatenliedern des Ersten Weltkriegs
  [ War and Speech: Studies of German Soldier Songs of the
    First World War ]

  Weigel, Hans - Jeder Schuss ein Russ, jeder Stoss ein Franzos :
    Kriegspropaganda in Deutschland u. Oesterreich 1914-1918
  [ Every shot a Russian, every thrust a Frenchman :
    War propaganda in Germany and Austria 1914-1918 ]

Gerhard Dangel-Reese
Augustinermuseum
Freiburg

 [ There is a significant difference between the WW1 songs the soldiers
 [ were singing, and the what the public thought they were singing.
 [ In France, as in America, the government encouraged the commercial
 [ songwriters to produce happy (and often silly) war songs, which were
 [ duly performed in the posh cabarets of the city but were generally
 [ hated by 'les poilus', the 'hairy ones' in the trenches in France.
 [ There is no happiness fighting a war.  The melancholy hit song of
 [ WW2, "Lily Marlene", was beloved by soldiers on both sides of the
 [ battlefields.   -- Robbie