Here is a helpful hint for those running x64 on some laptop models

If you try to use headphones and the onboard speakers don’t mute, you can turn them off by going to the audio mixer (run->sndvol32, or double clicking on the speaker)

Options->Properties->Adjust volume for: Other-> Line Out, and muting the line out.

 

This turns off the onboard speakers (but doesn’t seem to effect the dock speakers when the laptop is docked)