Is there any processing needed to playback audio?
This is unlikely to be a signal processing question. This is about the internals of the dsPic's firmware libraries and would be more suitable on another stackexchange site.

Things that could go wrong:


ADC input and PWM output might have a different data format (for example 12-bit right justified for the ADC, 8-bit for the PWM). You might need to scale / offset the data.
ADC_BUFFER_SIZE and OCPWM_FRAME_SIZE might be of different sizes, requiring you to use an intermediate buffer and counters to keep track of data pouring in/out of this buffer. Or, if the implementation allows it, you might instead read/write partial buffers of a smaller size - identical for both calls.