RR_NDB, what would "hi-fi" bring to the table other than a lot of noise that would have to be filtered away to hear the voice clearly? Surely as a ham radio operator you have played with filters to make understanding what is being said easier. Have you ever found that a 15 kHz wide filter could make a voice in noise sound clearer? Have telephones ever seen a need to go to 15 kHz audio response for general use?
Memory is cheap, today. But, the plane was designed when large amounts of memory were quite expensive and quite large. So having the electronics filter the sound the way it was is likely necessary.
With today's model recorders will likely store more - days worth? - rather than more bandwidth unless the bandwidth really buys "something" if applied to all the microphones. (However, I might store more channels to get information about wind flow around the cockpit and record them with 96ksps at 16 bits in the hope that might provide information that could diagnose cockpit noises as ice on the windshield, flying through rain, flying into a sudden up or down draft, etc.)