audio codec design
I think you need to start by reading literature about human hearing eg human ears can distinguish between different frequencys but are insensative to phase,
perceived loundness of a paticular sound pressure level varies with frequency. Loud sounds can mask quiet sounds. There are a finite number of nerve cells for different frequencys so there must be some limit to the ability to tell the different between frequencys eg 400Hz versus 400.1Hz
You will also need a good knowledge of mathematics such as fourier transform.