Which microcontroller for audio synthesis/processing?
Look at Microchip dsPIC family of microcontrollers as the features they offer may fit your needs.

The family offers the following features:


16 to 70 MIPS, DSP options
30 MIPS DSC at 5V
Intelligent Integrated Analog: Op Amps, 10/12/16-bit ADC, DAC
Integrated Codec interface
Motor Control peripherals




You can find more detailed about dsPIC30F features here: