I don't think stereo music listening has declined at all over the past 40 years. In fact, I would argue that the percentage of people who really, really love amazing musical reproduction experiences has expanded over the past 40 years.
What I believe is happinging is that there is always a pop-culture element to any personal entertainment technology which has always been shifting. From 1950 until the mid-80s having what was culturally recognized as a "high-fidelity" stereo system was considered a sign of success, made you more appear intellectual, and was a modern variation on asking a girl to come upstairs to "look at your engravings" (look it up). At a certain point in time, the cool technological toy to own shifted from a killer stereo to a primitive HT (BetaMax, LaserDisc, 25" Color TV) to eventually full surround sound systems, to today I believe it is the iPad and iPhone (I see more people bragging about their iPad and showing it off in public than I see people actually using them in public). That is the fate of technologies.
So, while the popularity of stereo waned when the pop-culture, want to be part of something, desperate for validation, trying like hell to get laid crowd shifted to video, big screen TVs (I remember listening to people brag about the total diagonal inches of their projector screens), pagers, mobile phones, smart phones, tablets, etc., --- I think those who have learned to love stereo music are a growing crowd.
Music can become so incredibly enlightening to the soul. It can provide solace, joy, peace, and lift one up and out of the day to day stress of work, life, and trying to get by in the world. It is accessable to all and no one class has ownership of its pleasures. When a popularity seeker buys his first iPod (or whatever cool technological music whiz-bang gadget is popular at that moment), he is exposed to something amazing. At first it is attention he craves, but over time having an experience with the music replaces trying to annoy all the other cars on the road by having the loudest subwoofers. Finding the spiritual escape becomes more important than the angry lyrics of a rapper or the boom, boom, boom of a synthesized nuclear explosion. Music collections shift from being tools to show off and impress others (and ourselves) to being methods of ethereal flight.
Yes, no matter how we got here - those of us who truly love music and the only true method for experiencing the raw, unadulterated forms of music (Stereo) ---- no matter how we discovered it: through the angst of youth, through the power and control of boom cars, through the whiz-bang excitement of technology, through whatever means --- however we got here, we love to stop the daily grind, forget about life and worries for a few hours, and get lost in a universe which only exists in sound, patterns, rhythms, melodies, poetry, and gorgeous connections with God.
