When singing or speaking, the vocal cords vibrate more than one hundred times per second as the air is forced through the windpipe. The vocal cords are actually muscles, more precisely two supple muscles in the larynx.

The sound of the voice is produced by the vibration of these muscles. Length and tension determine the different characteristics of a voice.
When imitating another voice, the vocal cords are made to change their tension. The lower the voice, the shorter and more relaxed the vocal cords are. High pitches stretch them.

As vocal cords are muscles, they can be damaged by overuse just like any other muscle. Extreme overuse can lead to the formation of nodules that have to be surgically removed and can leave permanent scars, i.e. changes in the sound of the voice.