sorry,
i didnt meant to call you a liar,
but fact is :
system needs specific events to happen to play pre-defined soundfiles, and if it does, the sounds are only part of much bigger workin-process...
like the sounds we talk about, they are defined for device connect events:
HKEY_CURRENT_USER\AppEvents\Shemes\Apps\.Default\DeviceConnect\.Current
so if there would be any voltage problems/differences from turning on lights in the room, that makes the computer starting a DeviceConnect process, playin specific sound, but what happens next??? process just doesent disapear??? maybe next thing in line is givin the device a name like F: or tryin to read devices invisible system files, and because there is no device, process comes to an error that results in displaying pre-defined error messages...
ohhhh, now i got it

your light switch also controls a seperate power outlet in the wall where you have a printer or scanner plugged in, something that is connectet to your PC, that can be the only possible reason...
EDIT:

ohhh BigMark and kman already said the same thing...
well sorry guys, time for me to turn my computer off and do something else, i am startin to go paranoid.... xD