I tend to think of the drums and pipes of armies in the 1700's through to Napoleonic times, as well as the songs of the American Civil War, yet somewhere between this and WWI they seem to disappear from armies, and even the creation of songs and music regarding the wars seems to move solely to the home front (although I guess this was always a major source). Perhaps it's just the rise of recorded music, but I'm interested to know any factors pertaining to the death of the marching band.
As a secondary question, if anyone would like to give a brief overview of how music was used in armies from early history to today I'm sure many including myself would be interested :)
At a recent tour at the Royal armoury in Stockholm I attended, one of the visitors asked the same question. The short answer is that the music was used to give commands. Remember that this was before any kind of radio, and the example the guide gave was that of Charles XII of Sweden. At the peak of the Swedish Empire, there was a large number of languages spoken, because of the large geographical area and the large amounts of POW's in the army: everything from Swedish, German to Polish and Russian. Using drums was far easier for everyone to understand.
To cut it short: mainly music was used in Charles XII army as a way of giving orders and at the same time bypass eventual language barriers. When radio communication was introduced, giving out orders with drums became obsolete.
Apart from the guide at the Royal Armoury, this book proved useful in answering your question: https://books.google.se/books?id=RBC2nY1rp5MC&printsec=frontcover&dq=military+communications+from+ancient+times&hl=sv&sa=X&ei=hj2NVNDeDeLCywO0-YDYCQ&ved=0CB8Q6AEwAA#v=onepage&q=military%20communications%20from%20ancient%20times&f=false see pages 306-308 for an example from the Boer war.
As a follow up question: When did (organized) armies stop using POW's as their own soldiers (join/die)?
Not representative of the war as a whole but during WW2 Piper Bill Millin piped the 1st Special Service Brigade ashore on D-Day in the face of significant and deadly defensive fire . http://www.telegraph.co.uk/news/obituaries/military-obituaries/special-forces-obituaries/7952729/Piper-Bill-Millin.html