Auto_ambient is a project mainly written in R, it's free.
Turn any piece of music into slow ambient music
Turn any piece of music into ambient music.
This function computes the spectrogram matrix of a signal, where each column corresponds to a frame and each row is an FFT bucket. Then a smoothing spline is applied for each row, horizonally, to create the ambient effect. Finally, the FFT is inversed and interpolated.
See the (well commented) source code for details.