Well, simple things first . . . are you sure that BOTH the wave slider and the Master Volume slider are raised? Having one or the other muted or otherwise silenced will cause no sound :-)
Aside from that, also make sure that Windows does not think you have a 5.1 surround setup. You can verify this by going to the Sounds and Multimedia control panel, under speaker setup. Make sure that it is set to 2 speakers, not 5 or 6 or whatever. The speaker out port on a Macintosh is a dual digital/analog plug, so you have to make sure it is not outputting a digital signal to your speakers.
