Enable tab audio muting UI control is a flag that has been available for Chrome on Windows for a short while. In the latest version of Chrome Canary - 40.0.2207.0 - we can now enable this flag on OS X.
The flag can be enabled with the following flag:
With the flag enabled you will see a warning signing when you hover over a tab which has audio playing.
If you then proceed to click on the tab the warning sign will be a much more visible and any audio from that tab will be muted.
You can also right click on a tab and the context menu will now contain a 'mute tab' entry.