No BIOS beeps

Put together a new system and everything seemed to work fine except occasional reboots when plugging in USB memory sticks. After about a week of operation I plugged in a USB hard drive and the system when blank. Could not start at all.
Removed graphics card, mem and CPU --> nothing, no beep, no power LED.
Checked the power rails, all seemed to be ok. Figured must be the Mobo. Send it back, got and replacement.
But still have the same problem.
Just Mobo in case with PS, no power LED no BIOS beeps
Rechecked voltages according to manual, all ok (-12V is -11.6V guess thats ok).

Could it still be the PS or am I missing something?
Help would be greatly appreciated!

Here is my system:
Asus M2N-SLI Deluxe, nForce 570 SLI
AMD Athlon 64 X2 5000+
Arctic-Cooling Alpine 64
Corsair TWIN2X2048-6400
Asus EN8600GT SILENT/2DHT/256M
Western Digital Caviar SE16, 7200rpm, 16MB, 320GB
CoolerMaster Elite 330 RC-330-KKN1-GP
Thermaltake TR2 RX 450 Watt, ATX 12V 2.0

Inside my case is a buzzer type speaker. Took a speaker (coil type) from another case -->still nothing.
I've even got my hands on another PS and got the same thing!
If I do not connect the AUX 12V (4 connector plug) I get the power LED lit when I try to turn on the system.
Of course it shuts down right away again.
I have a case fan connected and it gets it power correctly.
When I turn the system on, fan start turning but now power LED, no beeps.
Then I hold the power button for 10s and the fan and PS shuts down.
To me it sound like the mobo is still bad.

Just to make sure I'm not missing anything before I go yelling at the vendor and change to Gigabyte.
The BIOS should beep even if I don't install CPU. mem and graphics.

Thats what I am trying to confirm. Our PC guy told me that the BIOS should beep no matter what.
So which is the case, will there be any BIOS beeps without CPU or not, anyone knows?

The power LED is ok, checked that with a meter.
Tried with all Power switch, Power LED, HD LED, Reset switch and speaker connected or only power switch, power LED and speaker --> no change

Just for the record, the ASUS M2N SLS Delux DOES NOT PEEP if no CPU is installed, nor does the power LED go on.

I guess I was a little to cautious after I got my mobo back from the vendor (and had some wrong information)
After installing CPU, RAM and video my system works fine.
Thanks for everybody who helped.