Removing/reducing vocal volume using Garageband

Do you want just to silence the vocals, but keep the backing tracks?

GarageBand cannot unmix an audio file. and separate the tracks. The more expensive Melodyne can do that.

Or you could try the free Audacity. (https://sourceforge.net/projects/audacity/). It has a "Vocal remover " in the Effects menu. It works rather well for center panned vocals. This effect subtracts the left channel from the right channel, and since both channels contain the center panned vocals, the vocal track will be silenced.