notebook audio playback problem

[Liyu,She]

Hi, guys,
    I have some trouble with the audio playback.
    I am using Acer Aspire 4710Z notebook and it has a build-in speaker. So
when I play an audio the sound come from the build-in speaker.
    But when I plug my headphone into the hole the speaker still keeps
playing. The speaker couldn't be turned off from outside.
    Is there a way to silent the speaker when the headphone is plugged in?
    I am running FreeBSD 7.0 Beta3. The audio driver is:

FreeBSD Audio Driver (newpcm: 32bit 2007061600/i386)
Installed devices:
pcm0: <Intel 82801G High Definition Audio Controller> at memory 0xdc240000
irq 21 kld snd_hda [20071020_0048] [MPSAFE] (1p:1v/1r:1v channels duplex
default)

   Thank you.
        Liyu