Its likely that your speaker(s) are plugged into an outlet that's not on the same circuit as the rest of your system. Its also possible that those outlets are connected to a wall switch. You may think you're turning out a light, or some other appliance, and not realize you are turning off a speaker. Also, its fairly common to connect a wall switch to just 1 plug of a 2 plug outlet. It can be deceiving if you're not aware of it.
Occasional no audio on 2-channel system
My system includes a CD player, a preamplifier, two monoblock amplifiers, and two Martin Logan electrostatic speakers. The system occasionally has NO audio sound, or only in one channel works sometimes, and then one day they work like a champ with no issue. This drove me insane because I don't know what the problem is. I have tried to identify the problem, but could not, any advises would be very much appreciated.