As far as I know, ARM should be considered an architecture rather than a platform. However, the question is quite relevant as to what platform to use for RT signal (in this case audio) processing.
You could begin by asking following questions, not in strict order:
- How much time do I have for the implementation?
- What are my power constraints?
- What mathematical operations do I need? You might end up requiring lots of multipliers in parallel and hence limit your choice.
- How much memory do I need? (most MCUs are limited)
- [Important] What is my frequency of operation? How much can I squeeze in within the sampling period keeping operating frequency low?
- What libraries are available for my choice of implementation?
I would begin by looking at the algorithm first and foremost. If, for example, you need a lot of FFTs and MAC operations, you can probably rule out most microcontrollers and focus more on DSP cores. Bear in mind that there are MCUs with embedded DSP cores as well.
Another important consideration would be your ability and expertise in the area of implementation. Most people shy away from FPGA because you must use an HDL for the implementation. Another reason to shy away from FPGAs is the power requirement.