Sony cares too much about their own formats, which either make(Blue-Ray) or break (MD,UMD) . 7 years back I was the coolest kids in town carrying Sony MD around. Friends surrounded me every time I shove it out, thinking back it was hilarious... And now it's dead. Like very dead.
When you see how Apple market their iPod, it set the bar very high, and is very hard to beat that sort of influence. There is a good reason why everyone and his dog call every MP3 players iPods. Heck, even the government use iPod as a signature for any sort of portable MP3 players on the street and in tube station now adays here in the UK. I mean, it's a cultural symbol already.... now sony can try and beat that but it will be hard. Zune tried, and did manage to get hold of marketshare to some degree, but no where close to iPod's.
Back in the old days Walkman is the epitome of portable musics. Time changes since the introduction of MP3 format, and Sony doesn't seem to get the right direction. You can see their attemtps to create alternatives like MD players. Even then they still stick to their CD players as the main portable lines. I recalled it was already well into iPod 2nd/3rd gen that Sony tried to come up with a nice looking MP3 players, but it was too late probably.
