It's annoying when videos and audio clips start playing by themselves when you visit a web site. Here are a few ways to squelch them, via Kirkville:
For Chrome: Get the Disable HTML5 Autoplay plugin. This blocks both video and audio from playing automatically.
For Firefox: Use FlashStopper.
For Safari, there used to be an excellent plugin called ClickToPlugin, but it is no longer being updated. So to turn off autoplay videos, you need to first work in Terminal. Quit Safari, then open Terminal (it's in /Applications/Utilities). Paste this command into the window:
defaults write com.apple.Safari IncludeInternalDebugMenu 1
Press Return, and wait a few seconds.
Image: @Goga via Twenty20