Carols, of course. By 1915 all of the standards had been established, owing partly to a rise in the popularity of Christmas (and carols) in the English-speaking world following the success of Dickens' 1843 A Christmas Carol.

Examples of songs one might have heard at Christmas in England and America in 1915 would be Here We Come A-wassailing, The Twelve Days of Christmas; O Come, All Ye Faithful; and O Christmas Tree.

Songs you would not have heard in 1915 would be the English version of the Huron Carol (a 17^th Century Canadian Christmas song translated in the 1920s), Little Drummer Boy (1941), Mary's Boy Child (1956), Have Yourself a Merry Little Christmas (1944), Winter Wonderland (1934), Santa Claus is Coming to Town (1934), White Christmas (1940), Let it Snow! (1945), or any other of the dozens of 'pop' Christmas songs written in the 1930s-50s.

Broadly speaking, it was not until the 1920s and later that one would hear much in the way of Christmas music outside of caroling.

Sources:

• Rowell, Geoffrey, Dickens and the Construction of Christmas, History Today, Volume: 43 Issue: 12, December 1993, pp. 17 – 24

• Miles, Clement (1976). Christmas customs and traditions. Courier Dover Publications.

Don't forget "Deck The Halls" which had been around in various forms for a few hundred years. By the late 1800's the English version we know today had been published.