In episode 3 of the second season of Downton Abbey, Mary Crawley sings “If You Were the Only Girl (in the world)” to the piano accompaniment of her sister Edith. Mary laments that there are no available men to sing with her, as the song was meant as a duet. Written in 1916 for The Bing Boys Are Here revue in London, the song became a huge hit, and was originally performed by George Robey and Violet Lorraine (go here to listen to the original singers and to read the lyrics). The Bing Boys Are Here was considered one of the most influential musicals on the London Stage during World War I, its songs becoming forever linked to the period.
Soldiers often sang during the first world war as a way of boosting morale and to escape the misery of the front. Men might sing renditions of “It’s a Long, Long Way to Tipperary” as they marched, or “Keep the Home Fires Burning” while at base camp or in the trenches. The former was written in 1912 and was first sung by an Irish Regiment (the Connaught Rangers), and was soon adopted by many other British troops. The latter was written in 1914 specifically for the war.
Soldiers also amused themselves by taking popular tunes and creating new lyrics that often had an anti-war message. Tune names were changed to titles like We’re All Waiting for a Shell and Hanging on the Old Barbed Wire.
You can go to First World War.com for more songs (listed by year) with vintage audio.
Source: Canada and the First World War, Wikipedia.