You like old parts, don't you?
The old DS1669 digital pot is linear. A volume control is supposed to be logarithmic.
Maxim-IC have some logarithmic digital volume controls.
Looks like a couple of buttons is fine.
**broken link removed**
Of course pin 10 will be for analog input in your design.
What will the speech filter do? Remove the very important high audio frequencies of consonants and remove realistic low frequencies?
We can hear from 20Hz to 20kHz. Our hearing doen't make everything sound like a 300Hz to 3kHz telephone.
Modern hearing aids are programmable to fit the frequency response of the wearers ear. Could be a tiny DSP.
Wonder why the OP is building one? You'll never be able to make one as small as a professional aid. Why a digital volume control?
Did hearing aids ever use vacuum tubes?
This one will have a similar size and weight.
Hearing aids have frequency equalization. This project should include it.
A simple treble frequencies boost circuit will also work pretty well.
Why are you building this? Is it a school project?
Even with modern small ICs and other components it will be about the size of a cell phone (if your skilled)