Leadtek K7NCR18GM : BIOS beeps thru ext speakers.

I've recently assembled a system for a friend using a Leadtek
K7NCR18GM mobo and it works fine except for one thing : The BIOS POST
beep comes through not only the cabinet internal speaker, but also
through the external amplified speakers. If the ext speaker is already
on with the volume control set to a moderate level when the computer
is turned on, the amplified beep is loud enough to make one jump.

I've read the manual and gone through the BIOS settings, but can't
find anything to correct this. Trying to remember to turn down the
speaker volume control before each startup is not really the answer.
Did I overlook something ? Can anyone please help ?

-Zotin