Hi Tony.
Thanks for the explanation, it's helped get a better idea of how you're set up!
I think the problem is simply this: As you've only got four speakers connected to the Yamaha, whenever you play your DVDs or Blurays the film will be outputting 5.1 channels of sound but the centre channel will not be heard (this is the one that carries the vocals and is usually situated underneath or on top of the viewing source, hence the "centre" tag given to it!). Consequently the dialogue of any film at the moment will sound distant/muffled/unclear as the amp is being told to output on all 5 channels of sound, yet there's nothing connected on the centre channel.
If you're looking at options for connecting some additional speakers to the plasma so you can pop them out underneath it when you watch a film, then hide them away again for everything else, why can't you simply wire another speaker back to the Yamaha and connect it to the centre channel? You can still hide this speaker away when you don't need it but when you do watch films you'll hear everything crystal clear as this speaker will output the vast majority of the vocals. It's still underneath your TV so shouldn't upset the wife, and other than popping another speaker cable from the amp to the TV there are no additional wires to worry about (mains cables etc).
I think this would be the answer to your problems. When watching the TV and listening to it through your amp i.e. Eastenders etc, you've not noticed any issues because invariably the broadcasts are in stereo so there's nothing specific being channelled through the missing centre speaker. To change the sound output for a film, go into the top menu for the DVD/Bluray you're watching (usually a button called top menu or disc menu on the remote - it's the first screen that comes up when you load a film and has other options like Scene Selection, Extras etc). On that menu look for the option that will be called something like Audio Selection/Language Selection/Setup etc. Click on that and you should see some choices with regard to the sound for the film you're watching. The default for most these days is English 5.1. To change to a stereo output, just select the one that says Dolby Digital 2.1 or similar (basically anything apart from 5.1!). The DVD/Bluray will then output 2.1 sound instead of 5.1 and my guessing is that it will sound better almost immediately when you watch it. It's not a setting on the Yamaha or anything in the Sony menus that changes this, it's in the menu for the film you load at the time and as such would need to be selected everytime you watch a film.
If you were looking to get some speakers anyway, I'd definitely pursue the addition of the centre speaker underneath your plasma and wired back to the Yamaha. That should solve all of your listening issues in one hit, plus avoid any unnecessary cables and wires!
Unless I'm missing something, is this is ok or is there a reason why you never connected a 5th speaker to the Yamaha when you first set it up?!
Si