Thanks a lot guys for the nice input about microphones' response.
But I'm a bit puzzled. From my naive point of view I think I should not be concerned about that.
I mean, I'm trying to record electric signals directly from guitar/guitar amp / digital keyboard headphones output to my smartphone via the minijack mic input, so no transducer (with its intrinsic response) involved.
That said, maybe what some of you meant and I did not get is that the electrical freq response of the phone amplifier right after the minijack input is optimized for voice frequencies.
Well, in order to discard some potential causes of this white noise I improved the cable by: adding the stereo-to-mono passive adding circuit (10K R, 10K L and 100 K to ground), used a shielded cable and added a voltage divider with a potentiometer to attenuate the output signal from the device in the cable itself and not by reducing the volume of the guitar/keyboard.
Nothing really helped. Results were similar as in the audio attached.
Then I decided to remove more components of my system, and basically record the audio when having only a short at the input (I shorted the 4-pol mini jack GND to MIC)... and I still had the noise... so I'm afraid that the smartphone itself is limiting me... either the front-end electronics or the ADCs... does this conclusion sound right to you? Maybe I'm going into so much trouble when I could get way better results with a raspberry Pi and the additional ADC... and have way more flexibility.