Fixed wrong audio volume when initializing SC.AudioView with specific volume value

I noticed that when `SC.AudioView` is initialized with a `volume` value, it is ignored and set to `1` after the `loadstart` event has occurred (when using HTML5 audio).

Example: 

    audioView = SC.AudioView.create({ volume: 0.25, value: static_url('test.mp3') });

When calling the `play()` method then, the audio file is played back at full volume (`1`). I've added a test that fails with the current master and succeeds with this patch.

**Note:** The test doesn't work in PhantomJS since it hasn't audio support. It will issue a warning when run in PhantomJS.

Looking forward to hear your feedback!