Download LetThereBeSound and uncompress. Using the terminal, navigate to the newly-created folder. If you are running Linux, open Jack (using a program such as QjackCtl). Type "make" and hopefully everything should compile smoothly. To run the program type "./LetThereBeSound [param1] [param2] ..." The various parameters you can use are:

Additionally, you can optionally add an extra parameter "--input" immediately before specifying the wave type to modulate the input. Or you can just type "--input" without a waveform for the pure input signal.