Just want all to know that the BOOM application I have installed is still working well at correcting this problem. I have now watched several videos from various sources and applications, and none of them manifest the problem with BOOM installed and turned on. In fact, I own the "The Life of PI" and when watching in iTunes without BOOM on, the volume drops at the 4:29 mark. Once BOOM is back on, no fluctuation. Nice thing about BOOM is not only does it help boost the paltry volume of the MacBook Air, but it also allows one to pick a custom EQ setting for the mac (similar to EQ on the iPhone).
All of that said, I think the volume fluctuation is a software issue that will need to be addressed by Apple. If it were hardware, iTunes music player would also manifest the problem which is currently not the case for me. I had thought of taking my MacBook Air in for a return since I am still well within the 14 day period, but based on the experience of many who have done so, it appears the replacement models have the issue as well.