Hummm Sound from stereo speaker when used on inverter
Inverter don't give a pure sine output. There are some kind of inverter which gives pure sine output but those inverters are much more costlier than normal inverters. Normal inverters will output kind of modified sine wave which doesn't be same as sine wave that you get from the mains power supply.This might be a cause for your kind of humming sound from your speaker system. You can also notice this kind of humming sound from your ceiling fan when it's working in inveters. My Suggestion is that you can try to get  a pure sine wave inverter and can fix these humming issue.  You mentioned that humming sound goes off when you insert it to phone or PC. This because they still give some kind of output even if you don't play any music with it. They always give much more higher SNR sound output even though you don't play any music