Get one of these HSPC ATX Control Kit
and plug the speaker into the proper header pins on the mobo.
Boot the computer with everything plugged in. You should get one confirmation beep of a successful POST
Boot the computer with nothing connected except PSU, CPU, 1 sick of RAM and Video card. See if it beeps. You should get one beep.
If it doesn't beep remove the RAM and boot again and check for beeps. You should get multiple beeps for failed RAM.
Boot again with the VGA removed you should get beeps.
If you get no beeps with the RAM and VGA removed the your problem is probably the mobo, cpu, or PSU. Since neither PSU works it's probably the mobo or CPU and more than likely the mobo.
It's easier to grab a speaker out of an old computer, but better the kit than no speaker at all.