the problem is the mismatch of impedances, since you are surely taking the headphone output of the mobile, the volume of the mobile decreases, if it saturates anyway, you should use a potentiometer or preset of a high value, around 500k you place it in series, and several the cursor path until it no longer distorts, logically you will lose treble that you then recover with a ceramic capacitor in parallel, after you get the appropriate volume without distortion, measure the resulting value with a tester and replace it with a resistance of that same value by placing the capacitor in parallel, the capacitor should be around 470 to 1000 pf, greetings