I used to work with sound back in the days and looking into STM32 capabilities I actually wonder if I should attempt to create the basis for a DIY sound synthesizer. The idea is to use a Sound IO Hat with 1-2 channels, add a STM32F405 (or similar) and stack the Hat’s on a Raspberry PI 3 to create a multi channel synthesizer & mixer.
Raspberry PI 3 delivers quite awesome CPU power itself, but a M4 should in theory be capable to add sound effect processing in modules as it contains DSP alike instructions and capabilities.
The electronics in this case will be a MCU + analogue input and output amplifiers. STM32 contains ADC/DAC with 12 bit resolution, but I would in this case look for components with 16 – 32 bit resolution and target a sampling rate up to 64KHz (I think).