Low speaker volume

I have a new XPS410 that came with Vista Business Edition. It has the Sigma Tel High Definition Audio CODEC (whatever that means).  I have hooked up my old PolkAudio speakers (the ones that came with my old HP) but cannot crank the volume up beyond normal "speaking"  level.  I tested the speakers and they are fine.  The microphone output works fine -- I can crank up the volume as loud as I want. I called tech support; they modemed into my PC and could not determine the cause.  It sounds to me as if the speaker output is a "lineout" that needs an external amplifier, but this would be a pretty unsatisfactory feature if so.

 

I will greatly appreciate any help on this...Thanks!