I remember having had similar problems and breaking into a sweat every time, and every time it turned to be something stupid, like a loose phono connection on the sub, having plugged the lead from the sub into the "yellow" video socket instead of one of the LFE outs while using S-VHS to the display, or using the wrong wall wart in my collection...
And I'm no more stupid than the rest of us!
So check, and check again, all cables OK and plugged into the correct sockets?