Designing radios with gnuradio
When dealing with audio, a certain accuracy is required, and a possible approach is supersampling. In the decimation, it is likely that some averaging is done to increase the accuracy of the reading. The hardware required for the ADC conversion is also simpler that way.

The extreme of this approach are sigma-delta ADCs, where just a comparator is used to sample at a single-bit accuracy with a lot of supersampling, and then some clever logic is used to reconstruct the signal. The advantage of this approach is that, in short, linearity is much better and easier to achieve.

Besides, while it's true that Nyquist frequency is twice the maximum frequency of the sampled signal, in order to get usable values you need to go farther than that. More likely 5-10 times the maximum frequency.