Have you tried it with coax instead of the scart? I am just wondering if the scart in the Sony needs to see a switching voltage (I think it is on pin 8 in a scart connection, although it is so long since I did any TV repairs for myself that I am not certain) and the Dreambox isn't putting out one. I know the Sky box puts out a switching voltage if it is set up to do it, which will make some TVs switch to the scart input when a voltage appears on it, but I don't know about the Dreambox or indeed the Sony.
It seems strange that you get a picture via the scart but no sound. Without knowing either unit it is difficult to help.
Do you have to select the scart input via the remote control?
Has the TV got more than one scart and if so are you in the correct one?
Are you using exactly the same cable and input when you connect the sky box as you are when you connect the Dreambox? If you are, try selecting that scart on the TV via the remote while it is showing the picture but no sound. It could be that the scart is not fully "open" without being selected either manually or by a switching voltage.
The only way really is to slowly eliminate one thing at a time. Sorry I can't be any more help. I have tried to tell you all the things I would do if I was in the same situation. I would still make sure that the Dreanbox is giving PAL I output though just to be sure.
