Sudden noise coming from an app on your computer is a  growing  problem.
So  far,  there  are few good solutions short of searching through every
tab of every window, which can take a long time  if  you're  doing  real
work on your computer.  Here's what I've managed to find so far:

Chrome:

The "Windows" menu shows an icon at  the  right  for  windows  that  are
producing sound. (The icon is supposted to represent a speaker and sound
waves, but this may not be obvious at a glance.) If you  click  on  such
menu  items,  their  windows  will  be  brought  to  the front, and tabs
producing sound will have a somewhat similar (but clearer) speaker+waves
icon at the right, just before the 'x' that closes the tab. You can then
click on that 'x' to kill the tab and its sound, or click  elsewhere  on
the tab to bring it to the front of the window, where you can do with it
as you will.

Firefox: 

There are claims that a similar feature is in  Firefox,  but  I  haven't
been able to spot it.  Maybe it's too subtle for my eyes? Addition: Some
comments say that it only works for html5 video images.  This isn't easy
to  test, since with most apps, it's difficult to discover the format of
a video.  In any case, html5 is not yet the dominant video  format,  and
probably won't be for the foreseeable future.