How can I insert message bits into the frequency spectrum of an audio signal using Matlab? I have been trying to use the DSSS spread spectrum technique for audio steganography in MATLAB. I multiplied the PN sequence with the message bits. Now I want to insert this spread message into a cover audio signal. In the time domain, I split the cover audio into blocks and just added the message. But I want to do it in the frequency domain, by taking the FFT of cover audio. How can I do that? Which is the better way?
The FFT is just an implementation of the DFT. There's, mathematically, no difference.

The DFT is a linear operation. So it makes absolutely zero difference whether you add up two signals in time or frequency domain.