Music is a universal language that can stir emotions and bring people together. Music is often used at peace rallies and events to both inform and entertain.
At some rallies, a left-wing choir or musical group will perform to help stir things up a bit. Music can get people excited and enthusiastic before the march begins, and can help keep the momentum going afterwards. Some musical styles commonly heard at rallies include hip-hop, folk, and rock.
The Raging Grannies can be found in many cities around the world. The Grannies are group sof older women who sing parodies of popular songs by putting political lyrics to them, while dressing up in exaggerated "old lady" clothes, especially big hats. If your city has a local chapter of The Raging Grannies, you should definitely look into getting them to perform at a rally. They are usually more than happy to oblige.
Local singer/songwriters can be a wonderful avenue of talent to tap into. They want exposure for their work, and may write songs that are edgy and full of meaning -- exactly what you want at a peace event. Some many also perform cover versions of classic peace songs like John Lennon's "Give Peace a Chance" or many of Bob Dylan's older material, especially, "Masters of War."
Pre-recorded music is also commonly heard at peace events. Besides the aforementioned songs, the music of icons like Joan Baez, Peter Paul & Mary, and other 60's musicians from the first wave of the contemporary peace making movement will draw in the older crowd, while introducing these sounds to the younger folks. There are also modern performers, especially singer/songwriters, like Richard Shindell and Josh Ritter, who have a lot of intelligent things to say.
However, peace music is not restricted to folk and acoustic. Punk and hip-hop acts can be politically charges and inspiring - artists that come to mind include Green Day (especially from their American Idiot album), Chumbawamba (well respected in the anarchist community) Gauge the music to the anticipated demographics of your event, and find people in the know to suggest and provide tunes.
If you are going to have music at your event, it is important to have a reliable and hopefully portable sound system. A microphone, speaker, and some sort of small mixer, are essential. Sometimes a sympathetic business can loan equipment for the afternoon, or rent it for a small fee. Plan for a setup and tear down crew.