a function that can generate a random amplitude between 0 and 1.
return random(0,10000) * 0.0001;
If the amplitude stream must behave differently you should adapt the inner part accordingly,
e.g. read values from an SD card, a circular array, from your analogRead(), from the inner clock % some prime
example a simple sawtooth, every call is 1/1000 more that the previous until it reaches 1.0 then it resets.
static value = 0;
value = value + 0.001;
if (value >= 1) value = 0;