the CVR inputs are sampled at 8 or 16 KHz and thus can record sounds up to 3.5 or 6 kHz.
You get excellent quality sound at 8 kHz sampling (300 Hz to 3kHz, not 3.5 kHz). 8 kHz 8 bit uLaw or aLaw is the standard for landline telephones and as very reader knows that's more than acceptable (NB mobile calls are far lower quality). Going to 16 kHz gives a higher upper frequency ( 7 kHz ) but in general that's only useful for engineering analysis rather than voice clarity.
Where there is a big difference is the number of bits in the sample. This affects dynamic range and so say with 16 bits you can hear clearly the slightest whisper as well as the loudest bangs. 16 bit 44.1 kHz sampling is also well beyond the usual human dynamic range which is why it's used in CD recordings (fake golden-ear pedants not withstanding).
All of these figures are pretty irrelevant these days. 8 bit 8kHz sampling is 64 kilobits per second. CD quality is 705 kbps - a factor of about 10. This is compared to the storage increases over the past few years of factors of thousands to millions.