i have just got one of the cards today and had that same problem, but it works fine during normal use, only if i go into standby mode it beeps and dosnt come on.
from what i can tell, it's a safety issue, when the power drops, the card cuts out and sounds a warning that there isnt enough power for it to work safely.
i got a 600w power supply with 2 PCIe slots and it works fine, only when sleep mode dose it beep when the computer comes out of it. and i hear a faint dieing beep on shutdown.