Well the R3E does not have a buzzer/speaker built in, so all you have to do is disconnect the 'SPEAKER' cable that comes from your case which you have plugged into your motherboard (bottom right-hand corner).

As for the beeping, I'm not sure as I have never heard of it or experienced it myself. Have you tried a different PSU? Perhaps it's a built in alarm for when there is not enough power being supplied by the PSU.