For OS Choise:
Based on your requirements, we recommend you to use Yocto, which gives you more freedom to use the HiFi 5 as a general DSP, and provides more flexibility for software customization.
On the IoT Yocto Genio 700 EVK we support the HiFi 5 DSP through the open-source Sound Open Firmware (SOF) framework. We officially guarantee that the communication path from the SoC to the DSP via SOF works correctly. We also provide a porting guide that describes how to build and deploy the standard SOF firmware and topology:
Audio DSP porting guide
And running a C applications on the DSP is technically feasible, but it falls under project-specific customization. This requires you to modify the SOF firmware code and perform the integration yourself. You can refer to the official SOF documentation for detailed: