^When I was studing semiotics on college, one of my projects was set around the idea of subliminal messages (that hidden messages that you can't really notice at first, but that are noticed by your incoscient). So, I studied about the mechanics of those stuff people notice as hidden messages (reversed songs, faces or skulls on pictures, etc).
I don't really remember the name of the property, but human brain is programmed to find pattern and to look for a sense, a mean on everything all the time. This is how we are able to recognize a human face or similar stuff on different contexts. Sometimes, our brain trick us into finding meanings and patterns on irrelevant stuff. This is the case most of the time when we hear words in reversed tracks... It's like seeing figures on the clouds. The messages people find tend to be satanic or dark stuff cuz this is what people try to find. ^^
About the death images being about ageing... This is very used on western art (
http://en.wikipedia.org/wiki/Vanitas, thanks thisisLOL that explained this art concept to me and pointed how much Ayu uses it). The YouTube interpretation is interesting, but I tend to believe people like to overthink... It's such a complex concept that it would be too hard for most people to figure out by just watching the video... and when studing comunication, we learn something I tend to believe: If the message can't be received, it's the same of it not existing.^^