I want to use one of the ST dev boards to prototype this project. Having support for mBed, Arduino and GCC without having to purchase any programming tools makes the NUCLEO boards a no brainer for me.
The F303K8 is the smallest form factor board that supports DSP, so I'll plan to use it for now. The rest of the circuit is fairly straight forward. It basically consists of a way to get audio in and out of the microcontroller. An electret condenser mic with some buffering will feed the ADC input. On the output I'll plan to either bit-bang or PWM the speaker through a small N-channel FET. The circuit may (and probably will) evolve from this basic schematic, but it's a starting point and enough to start looking for parts.