Ihsan Kehribar has designed and built a Polyphonic FM Synthesizer with STM32F031 project:
I’ve built a playable, MIDI capable, polyphonic FM synthesizer using an 32bit ARM Cortex M0 microcontroller and an audio codec.
I used an ARM Cortex M0 core STM32F031 for the project. Cortex M0 cores don’t have anything really extra than having a 32×32 hardware multiplier for DSP purposes. Cortex M3 and M4/M4F have specific DSP instructions and they would have been much more helpful for such a computational intensive task, but I just wanted to try what can be done with a low cost, entry level ARM chip. That specific microcontroller also have I2S and DMA module which is essential for this project.
For the system clock, I’m using the internal PLL to generate 48 MHz clock but I left optional XTAL footprint for future possible use.
Project info at Kehribar’s blog.