The siren beep sounds to me like a CPU temperature warning, so it may be worth making sure its properly seated, the fan's powered and the heat sink's secure.
My first thought was that the memory was shorting, but if you've tried both some old and new memory sticks in the board then that's not likely. You could try taking the RAM out completely and trying to at least cause memory beeps. If you get no beeps then its something serious on the board, if you do get beeps then its something to do with the RAM.
I'd say to strip the board down to the bare essentials, and use a known good power supply, processor and single RAM stick. If it still fails to even POST, just take off the components, find out which ones work, and chuck the board itself.