Hi Stephen,
we should not actually hear the radio at all. Because we are moving faster than the speed of light in the animation.
Artistic licence allows these errors. The entire scene is represntative of moving away from Earth and perhaps stepping through time slots backwards.
Its to create the atmospere early on of what the producers want. Personally I don't believe it to be erronous at all. If the producers were to make it accurate, the opening scene would lose its effect. Being too long or too fast Zooming out of the Solar system in the blink of an eye.
The effect that the producers wanted has been achieved. If they maintained the time ratio the scene would be out of the solar system too quickly to take in the planets passing by.
Alternatively if they maintained some approximation of the speed that the radio waves would reach each destination the scene would be too long. Several hours in the Solar System.
What they have done is an excellent representation. Critics pointing out the inaccurasies is fair enough but with fore-thought you would see it is the best way to achieve the effects. Analyse the two time frames you will see what I mean.
Cheers