Look at shareholder value
What you need to look at is whether a takeover would bring additional value to shareholders over them just owning shares in both companies as in many cases they already do, and whether they can bring additional benefits to customers over them buying products from both companies as they sometimes do at present.
Take Disney Corporation. Steve Jobs already owns quite a bit of Disney. As far as I can gather, he actually owns a higher percentage of Disney than he does of Apple. Could a combined Apple / Disney offer anything beyond the current ability to buy Disney movies on the iTunes store? I can't really think of anything, and the other movie studios might be less willing to sell their wares via a competitor's store than they currently are via an independent third party's store. That takeover could actually destroy shareholder value. Also, AOL Time Warner shows that tech and media don't always mix.
With Sony, there are a lot of duplicate products, but is there anything to be gained by trying to combine MacBooks and Viao laptops for example. Not really as they run different operating systems so are largely targeted at different people. Steve Jobs is never going to ship a MacBook with Windows 7 installed on it, and a Viao running OSX isn't going to get any more customers beyond the current Mac customers.
The Walkman mp3 players could vanish and nobody would really notice. Those that do currently buy them probably wouldn't go for an iPod in most cases. The Playstation could be ported over to iOS. That could probably work. Sony Ericsson phones - apparently they are still around though they don't make much of an impression on me on my visits to Carphone Warehouse.
With the media side of things, the arguments are much the same as for Disney.
Adobe - that could work actually. A lot of people buy their Macs to run Adobe products. Apple could make their products run better on Macs than they do at present. Then again they could create their own products or buy something cheaper and drive Adobe out of the market with a better/cheaper/better marketed product in much the same way that Adobe drove Quark out of the market. Bill Gates would certainly do the latter.