Audio tags turn out to have inconsistent support accross browsers.
Chrome on Android and IE9 fetch the file from the server when a new audio node
is created even if the file is chached.
Safari on iOS implements a singleton media object (behind the scenes) which is
deleted and recreated when a different audio file is played.
This patch alleviates some of these problems by reusing the original audio node
instead of cloning it.
A drawback to this approach would be that multiple click or delete sounds
couldn't play, however, iOS and Android don't support multiple audio tags
playing at once anyway.
So, we are really only hampering IE9, which lags anyway without this fix since
it refetches the file from the server upon node creation.
I thought it was better to provide audio feedback for most UI events and miss
the few that overlap rather than have every sound lag by 1 second.
A second drawback is that we are modifying a property (the volume) of a CONSTANT
object. Finally, this is not a complete fix since Android still lags when
switching between two different sound files.

R=neil.fraser@gmail.com

Review URL: https://codereview.appspot.com/11487043

git-svn-id: http://blockly.googlecode.com/svn/trunk@1108 c400ca83-b69d-9dd7-9705-49c6b8615e23