As far as I know, Void Elves are always hearing whispers from the Void in their minds.
And as far as I know, Death Knights are also always hearing the Lich King in their minds, whenever it suits the LK to do so.
How does it works out?
I can only imagine things like this happening in the mind of a Void Elf DK:
Void: Kick that kid, Do it!
Lich King: No, I don’t allow it.
Void: Don’t listen to him, do what I said.
Lich King: Yo, I’m your boss, not da void
Void: But I’m cooler, Are you going to do what I said or not?
etc
I think I’d lose my marbles if I had two voices screaming at me to do things, let alone if both voices argued over how hard I should punt the gnome before me
The key is “whenever it suits the Lich King”. Even Arthas in his pre-merged-with-the-Lich-King days only heard him when it was really important. These days, I doubt Bolvar is interested in bugging any of the thousands of death knights under his command.
I also really doubt that the Lich King could hear the Void voices talking (or probably vice versa).