The computer only "beeps" once if you have a small speaker connected to the speaker pins on the motherboard, either from the case leads or some cases come with a tiny speaker in the hardware package. Connecting a speaker to the back sound port on your backplate doesn't create the "beep" sound.
You have to connect the motherboard speaker if you want to hear the motherboard beeps. That's not unusual. I left my motherboard speaker disconnected on purpose, because I don't need and don't want to hear weird beeps. The speaker is a small one, connected to the motherboard by 4 pins.