Simple playback audio with FFT not working (dsPic33F)
Perhaps it's an outdated answer. Anyway, the issue with your code is that FFTComplexIP store the result in the bitreversed order. You must add the 

BitReverseComplex(7, &amp;Frame[0]);


between the FFT and the iFFT function calls.

The following is written in the dsPIC Language Tools Library:


  Remarks:
  N must be an integer power of 2.
  The elements in source complex vector are expected in natural order.
  The resulting transform is stored in bit reverse order.