Question about MCU's, and Audio Manipulation
For doing audio filtering, I recommend using a DSP (Digital Signal Processor).  I wear hearing aids and know there is a DSP hidden somewhere inside the case. Mine are the kind that have a minimal speaker unit that fits in the ear, and the electronics sits on the top of my ear.  After an audiologist performs a hearing test, the results are uploaded to my hearing aids where it programs a graphic equalizer.

Microchip makes a line of chips that combine a DSP with one of their PIC24 microcontrollers. For example, the dsPIC33FJ128GP802 is available in a 6mm x 6mm QFN-S package.  It is also available in a 5 mm x 5 mm VTLA package, but Digi-Key doesn't appear to stock it.  You can also get it in a DIP package, which would be useful for prototyping.

Microchip has two libraries available for designing filters, a Digital Filter Design for $250, and a Digital Filter Design Lite for $30.