[quote]How will I pick the audio signal (it is analog in nature) using SPI interface ?[/quote] That doesn't go together. Either with the internal ADC, or use an external ADC with SPI interface. But honestly, I would suggest to start smaller. To manage the audio sampling, processing, formatting (WAV ?) and file system operations in a concurrent manner under real-time conditions is guaranteed to overwhelm and frustrate a beginner. Not to mention that I agree with Brian - the selected MCU seems not quite up to the task.
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