It's called the "Asus Post Reporter". It's on several motherboards, and is meant to replace confusing beeps. So, instead of getting a beep code that you'd have to try to figure out, you instead get an actual voice error code.
There should be an option to toggle it off in the bios setup. If toggled off, you should get beeps instead of the voice. The real issue is to determine why you're getting error messages to begin with.
There is another option. You should have a utility called the Winbond Voice Editor that came with your motherboard CD that will allow you to customize the voices that are emitted from the built in chip on the board. You can even toggle some off, apparently, and then have the chip re-write itself (which takes a good deal of time; a slow process). Or, you can record your own error messages if you want.
I think if you want to prevent a certain message from working, you clear the text of the message with Winbond, or rerecord silence over it.
From what I understand, the voice chip is wired to the motherboard's line out sound jack. If you are using your own soundcard, chances are this is why you are not hearing the messages well. You might want to temporarily connect your speakers to the motherboard's built in line out jack to hear exactly what message is being said when you come out of standby, etc.