the manual says, in UEFi's boot tab, there is an option called Boot Beep. "Select whether the Boot Beep should be turned on or off when the system boots up. Pleasenote that a buzzer is needed." Yeah, great idea, because this mobo has no speaker header!
Is the 4-pin ATX12V1 connector near the CPU inserted? Did you try to boot without the R9 280 inserted and the monitor connected to the board? Some mobos won't boot the first time before a cmos clear is made.