In some fantasy literature, fiction dictates that certain warriors (usually tribal or barbarian ones) use chanting either to moralize themselves or to demoralize the enemy.
Now, I know of marching drums and such, but are those demoralizing chants limited to fiction, or do we know of any warriors and soldiers who made use of music in order to lower the morale of their enemies?
According to Plutarch, something like this happened at the Battle of Aquae Sextiae (Marius 19, 4);
However, though their bodies were surfeited and weighed down with food and their spirits excited and disordered with strong wine, they did not rush on in a disorderly or frantic course, nor raise an inarticulate battle-cry, but rhythmically clashing their arms and leaping to the sound they would frequently shout out all together their tribal name Ambrones, either to encourage one another, or to terrify their enemies in advance by the declaration.
As for music, there are modern instances of it being used to demoralize or as part of psychological warfare. For example, the Soviets in WW2 played Argentinian tango music on loudspeakers because they thought it sounded sinister, and during the US invasion of Panama they played hard rock music (including Van Halen's 'Panama') outside the Holy See embassy to harass Manuel Noriega until his surrender.
EDIT: Fixed formatting on quotation
The Hussites have sung hymns and chants before battle, "Ktož jsú boží bojovníci" being the best known example. We actually have a useful and valuable source for many of these religious songs, the Jistebnice hymnbook, though we lack any reliable evidence for their supposedly terrifying and demoralizing effect on the enemy.
Contemporary chronicles do mention that during the fourth Crusade against the Hussites, at the battle of Domažlice, the German troops "have heard the Czechs march, and in great fear fled before even seeing them". In popular history, this is often portrayed as the Crusaders retreating as soon as they heard the Hussites sing their hymns.
However, it is more probable that the speed of the attack coupled with the lack of organization and miscommunication in the crusaders' camp were to blame for the quick Hussite victory rather than singing alone. As parts of the crusading army were pulled back to consolidate their lines, other units mistakenly interpreted this as a retreat and fled.
Regardless of whether their voices were a terrifying weapon or not, many of the Hussite songs remain a part of religious tradition to this day, and "Ktož jsú boží bojovníci" has taken on strong nationalistic connotations – much like the Hussite movement itself – over the centuries to this day.