to understand why having zero crossings at the start and end of a looped sample isn't necessarily going to prevent an audible click, it might help to know why the click is audible in the first place.
to convert the audio signal to sound pressure waves that your ears can detect, the transducers in your speakers or headphones pulsate back and forth, following the signal. a click occurs when the transducers are unable to follow the audio signal without causing lots of distortion. when is the signal hard to follow? it's when the signal jumps too rapidly from one discontinuous level to another, interrupting the wave-like motion of the transducer.
so, for example, if the signal is rapidly ramping down from a positive amplitude at the end of the loop, and the start of sample is rapidly ramping up to a positive amplitude, then that may cause a click, even if the loop starts and ends on a zero crossing. in other words, the speaker cone is racing towards the zero crossing, and then is then being asked to abruptly turn 180º and head back from where it came... click!
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