1978: Philips debuts a prototype CD player
1979: Sony releases the Soundabout portable tape cassette, later renamed the Walkman
1982: First commercial CDs sold
1984: Sony releases the Discman portable CD player
1990: Sony releases a digital audiotape Walkman
1992: MP3 developed. Sony's Minidisc first to use MP3 encoding
1994: Sandisk releases first compact flash memory card
1997: MPMAN releases first portable MP3 player in Europe
1999: Napster and file sharing explode
2001: Apple introduces the iPod
2005: Apple introduces the Shuffle, the first flash memory iPod

I got the above list from the Aug 06 issue of Fortune, which has a nifty gatefold pull-out of key developments in audio tech going back to 1965, plus schematics of portable devices dating back to Panasonic's Dynamite. Well worth checking out if you're into that sort of thing.

It's also got some interesting stats:
In 2007 dollars, CD sales peaked at more than $US15 billion in 1999.
It's been all downhill since, with CD sales slipping under $US10 billion for 2006.

Things aren't so grim at the mill as Fortune's timeline makes out.
Young 'uns may be attuned to never paying for anything on the net. But they'll line up like lemmings to buy cellphone downloads, and record companies clocked up more than $US2 billion in ringtone revenues last year. And on top of that, more than 3 billion songs have now been downloaded via iTunes alone on the street-legal internet download scene ...