The rule which states that a digital sampling system must have a sample rate at least twice as high as that of the highest audio frequency being sampled, in order to avoid aliasing and thus reproduce the wanted audio perfectly.
Because anti-aliasing filters aren't perfect, the sampling frequency has usually to be made slightly more than twice that of the maximum input frequency — which is why the standard audio rate of 44.1kHz was chosen for a nominally 20kHz audio bandwidth.