Unless the sound is actually starting to create problems you don't need to replace them.
My new philosophy with hardware is this, If both your top end cards can keep up with 1 top end "Newer" gen card then there is no need to replace them.
When they start struggling to get above 40FPS in your most played games then it's time to start thinking about newer tech, Until that time comes which I highly doubt is any time soon, Enjoy the technology you have