Hisses (also referred to as 'white noise') usually comes from random variations in amplifying transistors and/or circuitry; 'hums' are usually induced 'pick-up' of the 60Hz field (or one of its higher-order harmonics) that, due to our pervasive use of electrical wiring, are present all the time, nearly everywhere. Many 'hums' turn out to be 'circulating' currents in grounding circuits; grounds are meant to 'branch out', like branches from a tree-trunk. If there is more than one path to a particular ground point, a potential difference develops, which can make a path for stray currents (and it doesn't take much of one, to either disrupt digital circuitry, or induce audible noise) to 'run around' in a circle - and as nearly every digital operation depends on 'comparing' a signal to ground (is it 'on', or is it 'off' - the faster and more sophisticated the circuitry, the smaller the difference between the two), bad grounds can cause all manner of ill behavior!
voltage/amperage (I know I'm mixing up those -- I never did sort them out from physics class)
An easy way to think about it: think of water flow in a garden-hose; voltage is the water pressure 'pushing' the flow; current is the actual volume of water flowing...
Sound setups in comps are usually done with ICs that are specifically made to convert a digital 'stream' of pulses, conveying carefully formatted digital 'descriptions' of the sound to be made, into an analog audio signal, which is then amplified to the level required to drive (whatever - headphones, speakers - the like...). It is unlikely (but not impossible) that the incoming digital 'information train' is being disrupted, as then the noise would likely be bizarre, and not resemble the intended output. Somehow, adding the memory is likely causing some kind of 'leakage' into the amplifier section or beyond - what I can't think of (so far) is any real-world mechanism to explain
how!