with this extension we can obtain the peak value, the rms value, the decomposition of a signal through FFT and the waveform emitted by a Player, obtain and change the sound volume.

Instead of representing the graphical result of the waves in the time domain, it is represented by traces in the frequency domain. Each trace represents the amplitude of a wave and its frequency.

The decomposition of any signal usually generates an infinite number of sinusoidal signals.

If we add all those sinusoidal waves, we would obtain the initial signal.

If we add the first sinusoidal waves, we would obtain the initial distorted signal.

If the initial signal were formed by a certain number of pure sinusoidal signals, in its representation we will have that certain number finite of traces.

In addition to the graph of Amplitudes in the frequency domain, we can also obtain another graph with the phase shifts of each of the signals, we will not see that in this tutorial.

FFT is used in telecommunications, sounds, music,...

This is very basic information, check out other tutorials on this topic.

Thanks @vknow360 for the quick replay. You mean that I must build an extension based to TarsosDSP. I'll give it a try.
But, is there any easy/easier way to use mobile's microphone instead of the audio file that is emended in the @Juan_Antonio app, as input to the fft ;