I'm going to see if I can establish my credentials as an historian in
some field other than music by taking our esteemed moderator to task.
While Dave was playing the high end of audio with Marantz equipment
(tooling around in a Corvette, were we, Dave?), this writer was scrabbling
around with Dynakits, so this post is partly a sociological grudge match.
According to Dave, speaking of the year 1973, "Stereo was then 15 years
old".  From the standpoint of widespread availability of commercial
recordings for the consumer market, our distinguished moderator is correct:
widespread consumer use of stereo recordings began in the late 1950's.
Although there was consumer stereo equipment avaiable in the early 1950's,
it received limited commercial acceptance.

However, there is more here than his comment might suggest.  It had been
known since the early 20th century that recording music with multiple
microphones and replaying it with multiple speakers had advantages over
monaral recordings.  Indeed, engineers at Bell Labs were working on
multichannel recordings in the 1920's, but I think their work was limited
to headphones (which still have advantages over loudspeakers).  What we
today know as stereo is generally credited to a British engineer, Alan
Blumlein, who worked for EMI.  Historians of technology debate the relative
contributions of Blumlein and his American counterparts at Bell Labs
including Arthur Keller, but I think there is no debate that Blumlein
received the signficant first patent.

In 1933 Blumlein received a patent for what became known as stereophonic
recordings, or stereo (what he called binaural sound).  But I think the
honor of the oldest extant recording that could be called stereophonic
belonged to the Philadelphia Orchestra under that inveterate tinkerer
Leopold Stokowski, when they recorded Sciaban's "Poem of Fire" at the
Academy of Music in 1932.  The "Phillies" beat their British counterparts
by a year or two, as Blumlein waited until his patent had been approved to
engage in a public recording session (not certain that wait was intentional
on his part).

I think the first commercial application of stereo sound was Walt Disney's
film "Fantasia" in 1940, again with none other than Leopold Stokowski at
the podium, predating Dave's claim by over 15 years.

It is worth noting that Blumlein did not tie his invention to a two
speaker system.  Many developmental systems used three and four speakers
and microphones, even back in the 1930's and 40's (in fact, experiments
with up to 80 discrete channels were done).  The accepted wisdom was that
three microphones and three loudspeakers represented the best compromise
between high-quality imaging and practicality.  However, as companies
readied "stereo" for the consumer market, they realized that more than two
speakers would be prohibitively expensive.  So the "surround sound" and
"quadraphonic" systems of later decades were not plowing new ground in a
technical sense.

The early years of audio were populated by numerous curious and capable
people (well I guess that's true of the later years also, even though
the mass market is directed by "suits" who often know little of the
technology they use or the music they sell, but I digress).  Blumlein was
one of the best, and during WWII he assisted the war effort by working on
the development of radar.  Sadly, he died in 1942 in his late thirties in
a plane crash while testing a radar system.  Here are a couple of links for
those interested in this exemplary inventor who only in recent years has
gained the recognition he deserved:

   http://www.musicbooksplus.com/books/fp251.htm
   http://www.doramusic.com/chapterthree.htm

Larry