In Revelation, martyrs sing before martyrdom and after martyrdom (Revelation 14-15). Their martyr songs are the war songs of the Lamb, the triumphal chants of those who overcome because they, like the Lamb, do not love life even to death.
It doesn't happen only in Revelation.
In his comments on the heavenly hymn of Revelation 4 (Revelation, 89), Ian Boxall observes, “Throughout history, religious and political minorities have sustained their vision and found renewed strength through singing. The participation of vulnerable Christian congregations in the worship of heaven - not least through angelic hymns - offers them just such a vision.”
Alan Boesak saw this principle in action in the anti-apartheid movement in South Africa: “Black people in South Africa have made freedom songs part of the struggle; in fact, the struggle is inconceivable without them. Marching down the streets, facing the police and army troops of the South African government, they sing. In jail, they sing—songs of defiance and faith and freedom. . . . We sing because we believe, we sing because we hope. We sing because we know that it is only a little while, and the tyrant shall cease to exist” (Comfort and Protest).
Want to prepare people to survive, resist, and triumph? Teach them to sing.