there's no converting to go from DC-DC.
Best bet it to connect it to your PSU if you have the avaliable power on it.
Else, you can buy a point-of-load for the screen if your power supple will support them.
As for your speakers, that's a ground issue that could be any number of things. For that, please see this thread: Induced noise/Ground loop trouble shooting guide.
So are you saying then that the screen does have an egg on the power line?