Why Music Really Is Getting Louder
I don't use earbuds because I simply can't hear anything through them when used at a reasonable volume level outside of quiet environment. I'm inclined to believe no one else can either as evidenced by the sound I hear blaring (sometimes loud enough to be heard right through my own circumaural headphones) from everyone's buds on my daily commute.
Since the start of the iPod era, earbuds have become the standard for portable audio. Prior to that, only the cheapest of devices shipped with buds as opposed to phones. I would imagine this trend is a huge contributor to the loudness problem.