The first thing to establish is how old is the TV that is giving the problem? If it is older than about seven or eight years it is fairly unlikely to be multi standard, as the auto switching chips only started to be used in anything less than very expensive TVs at around that time. If it is older than seven or eight years it is most likely to be single standard PAL I and your problem is not the set but the signal standard you are sending to it.
As the manufacturers realised that it was easier to build models of TVs that could be used anywhere, instead of different models for different countries, the multi standard TVs became more or less the norm. So, today, almost all TVs are multi standard. Even a cheap one I bought off ebay last week does PAL, Secam and NTSC.
If your TV gives sound off of a Sky box then the sound system in the TV is okay and working with PAL I.
Assuming that you are feeding the signal in to the TV in the same way with the Sky box as you are with the Dreambox, using the same cables etc. then the problem is more likely to be the output from the Dreambox.
Your Dreambox is German, therefore I would guess that the default output is PAL B/G. If you haven't set the output to PAL I that is more than likely the problem. Or if you have done a factory reset for some reason the box may well have gone back to the default output standard.
Before you do anything else, make sure the output of the dreambox is set to PAL I, like Tivu said, otherwise you will not get anywhere. Please let us know how you get on with the Dreambox, and remember the old engineer's saying: "When in doubt
". Read the flippin manual!