here's a site

http://www.firstworldwar.com/audio/index.htm

that is pretty authoratative re: the history of the war, & with mp3s of songs of the war, dated when recorded. 'Land of Hope and Glory' stands out from 1912, would certainly have been known to the troops in 1914, (Pack Up Your Troubles in Your Old Kit Bag and) Smile, Smile, Smile! is 1915, from all accounts of the event other songs were sung, but they began with carols and ended with Auld Lang Syne. Another recent song is John McCutcheon's 'Christmas Truce of 1914' [Christmas in the Trenches] recorded in 1984. I am amazed by how often this idea of yours seems to have been done, from Canada to Lancashire, etc. there are websites recounting sing alongs and theatrical productions based on this event