They are provided to attenuate out-of-band signals (i.e. limit aliasing). We call the IIR filter the “voice” filter, as it is intended for lower sample rates (8kHz-16kHz) and has a sharper rolloff and greater stopband attenuation. This is necessary for lower sample rates because there is typically more energy above Nyquist for lower sample rates (if your mic and analog path are flat to 20kHz but your ADC bandwidth is only 8kHz, anything picked-up above 8kHz could be aliased by the ADC). The “music” filter is FIR in order to improve fidelity (FIR filters are linear phase) for higher sample rates that don’t require such strong out-of-band rejection.
Specifications for the filters can be found in the EC table on page 20 of the datasheet, specifically in the Record Path Voice IIR Lowpass Filter (MODE=0) section.
“Ripple limit cutoff” is the frequency at which the signal amplitude changes by more than the allowable passband ripple (shown as ±0.1dB); -3dB cutoff is self-explanatory; stopband cutoff is the frequency at which the signal is attenuated by the specified stopband attenuation (shown as 74dB). The exact filter parameters or shape of the filter is not shown, as it is not particularly important.
While we recommend using the “voice” filters for 16kHz sampling and below, and using “music” filters above this, this is not mandatory. Should the user want a steeper rolloff below Nyquist, they can absolutely use the “voice” filters at any sample rate.