Macbook Pro 15" bad hissing sounds when connecting external monitor and external speakers
What you're hearing is just electrical noise interference. The reason it depends on what's on the display is probably because it takes the display more power to show white than black.

To fix/help this problem:


Make sure that everything is plugged in all the way to its proper connector.
Make sure that you're using high- or
medium-quality cables, connectors,
and other components.
Use an external sound card like this one or USB speakers like these.