It's an almost incredible situation. Firefox and Safari have a setting to stop autoplay, but much of the time it doesn't actually work, and Chrome doesn't even seem to have a setting.

It's also amazing how when enough users decide that they hate some obnoxious web site behavior (pop-up windows, autoplaying video, etc.) enough to block it, the web site designer's response isn't "maybe we should be less obnoxious" but "how can we circumvent the user's preferences?"