Support for audio is always a bit different on each operating system I use, even when running on the exact same laptop. For example, with Windows Vista when I plug into the headphone jack the laptop's built-in speakers are automatically muted. That is not the case with Ubuntu 8.04 or OpenSolaris 2008.05.
The audio chipset is an Intel 82801H and while I can get sound out of the speakers when running Ubuntu 8.04, I cannot seem to get the headphone jack to work. :-/ With OpenSolaris 2008.05 I have had much better success. The speakers work and so does the headphone jack. But when I plug into the headphone jack the speakers are not automatically muted.
The fix is easy enough. I right-click the volume control on the toolbar and select Open Volume Control:
That brings up a dialog that shows that Line Out is on:
Turning Line Out off turns off the speakers.
And there is also a command line utility in OpenSolaris that will accomplish the same thing: audioplay. It is similar (but not identical) to the play command from the sox package. The audioplay command has a -p option that allows you to specify headphone, speaker, or line.