I would suggest you take out the mobo from the case and test it outside.
First, connect the power, speaker, & CPU. Turn it ON. If you hear the beeps sound, then you can add the RAM back to the mobo. Be sure to turn OFF the system first. Next the video and then HD. You add component one at a time to determine where the problem might be.
If everything works outside the case, but not inside. The maybe the grounding problem that cause your system not working.
This is not a clever way to do, but will get you started to troubleshoot.
P.S. Post beeps reference: http://www.computerhope.com/beep.htm