Let's say you had two 1 second sound samples at 44.1khz, and you wanted to convolve them in frequency space.
You FFT each one to get two arrays of complex numbers that each have 44100 items in them.
You then multiply each bin... for i = 0 to 44099 c[i] = A[i] * B[i]
Then it's time to turn it back into time domain samples. If you do an ifft you'll get 44100 time samples out, but when you convolve these two audio samples in time space, it'll give you 88199 samples out (44100+44100-1).
Obviously getting 44100 samples out is incorrect. At what part in the process did I go wrong? (: