Sound is an important part of the landscape in Mother Courage. The characters sing songs, but we also hear cannons, a funeral march, and most importantly, we hear a lot of drumming. And all this drumming proves to be pretty significant.
Kattrin's drumming is what signals a town of the approaching Catholic army, saving the townspeople but costing Kattrin her life. Though Kattrin can't speak, we could say that the drum gives her back the voice that was taken from her as a child, when she was attacked and raped by a soldier. Check out this exchange, in Scene XI:
THE ENSIGN: We'll have to set the farm on fire. Smoke her out, that's it.
THE PEASANT: It wouldn't help, captain. If the townspeople see a fire here they'll know what's up.
Kattrin has again been listening as she drums. At this point she laughs.
THE ENSIGN: Look at her laughing at us. I'm not having that. I'll shoot her down, and damn the consequences. Fetch the harquebus.Three soldiers hurry off. Kattrin goes on drumming. (XI, 162-171)
As the soldiers fail to find a way to stop her drumming and inadvertently aid her effort to alert the townspeople, there seems to be a moment of realization for Kattrin. Now she's the one in power. The drum might not allow her to speak words, but it does allow her to laugh in the face of military violence.
If we think back to Scene VI, when Kattrin is attacked while buying wares for her mother's wagon, we notice that the drum is among the things she brings back with her (VI, 234). So, the drum is not just symbolic of Kattrin's voice in Scene XI, but first enters here as a symbol of resistance against attack.
And if we remember Swiss Cheese's execution, we'll note that it's also the sound of drumming that signals his death off stage. Watch this:
YVETEE comes running in: They won't do it. I told you so. The one-eyed man wanted to leave right away, said there was no point. He says he's just waiting for the drum-roll; that means sentence has ben pronounced. […]
MOTHER COURAGE: Tell him I'll pay the two hundred. Hurry! Yvette runs off. They sit in silence. The chaplain has stopped polishing the glasses. I reckoned I bargained too long. In the distance drumming is heard. […] (III, 691-703)
Mother Courage appears completely helpless, as we all hear the drumroll announcing Swiss Cheese's death sentence. But she's not the only one who's helpless to stop the violence. Earlier, when the two men first arrive to arrest Swiss Cheese, Kattrin is unable to warn him because she can't communicate, though as the stage directions indicate, "She does everything possible to make him realise the danger" (III, 424-425). When Kattrin drums to save the town from attack, she is also breaking free from a position of helplessness. She turns the drum from a signal of murder into a symbol of hope and defiance.