The first edition of the Walkman was released way back in 1979 in Japan. It was selling by the name of SoundAbout in the U.S. and Stowaway in the U.K. at that time.
Over 30 years, Sony has sold more than 200 million units of this device, which is now truly a legend.
Incidentally, the Apple iPod, just completed 9 years of being around, this month itself.
Technically, the CD Walkman which was launched quite a few years back by Sony, was the successor to the cassette walkman. However, it is the success of the iPod which has changed the way people carry their music around.
Link to my original article- http://goo.gl/FP5y