For understandable speech, you need a bandwidth of 1.8 Khz. 2.1 Khz would be better and completely adequate. 2.7 Khz would sound more normal to you. Of course with normal hearing, more is better.

The "higher" frequencies make a big difference in understandably, especially with female voices. This has nothing to do with the fundamental or base frequency.

My hearing drops off quite dramatically at about 1.5 Khz. This is not quite enough to understand women and children well. I'm investigating various hearing aides. The modern ones all have essentially something like an audio equalizer (like perhaps on your stereo?) where individual frequency bands can be boosted or attenuated.

I would suggest that your family member go get a hearing test. COSTCO for example will do this for free because they want to sell an expensive hearing aide. The graph of hearing vs frequency will be of a big help in designing software that will actually help.

As an extra comment, when I was in my 20's I could actually hear the 20Khz sonic gizmos that turns lights on/off automatically and they drove me crazy. Not so anymore, but I miss that.

Update: I can't access my old machine right now because the M/B died. However, I remember the sound card having an equalizer function in its driver. It could be that just buying a better sound card than what you have is "the answer" without Perl or any programming required?

Update 2: Because of my own hearing difficulties I've been looking into this further. Apparently there is some freeware for Windows, Windows Equalizer that looks very promising. I have just started playing with the Raspberry Pi and I think there is some software for that critter. Also note that high quality noise cancelling headphones can do wonders versus the speakers.

So far, I haven't found any Perl solutions to your problem. That doesn't mean that there isn't one, just that I haven't found it yet.