I've had failure to POST twice. The first time was a faulty motherboard, the second was a faulty graphics card.
Try with just the motherboard and processor with heatsink, the power supply, case speaker and power switch. Remove the motherboard from the case and rest it on an insulating surface such as a sheet of card. The motherboard should now complete POST and give an error beep because it has no memory or graphics card. If it does you can start to add components until it fails to complete POST. If it doesn't, then one of the few remaining parts is faulty.