Tracey,
Our whiteboard requires 2 connections in order to use the speakers and the whiteboard. Plugging into the USB connection only gives whiteboard video and interactivity. For sound to work, we plug a 3.5mm stereo sound jack from the computer into the speaker control panel on the side of the whiteboard.
Actually, we do plug into the white wallbox, but that in turn is connected both to the speaker control panel and to the whiteboard.
So you'll need to buy a male/male 3.5mm stereo jack in addition to the USB cord that you're already using. I know, its redundant, and they don't tell you about that upfront, but it does work.
Anyone at Promethean have a better suggestion? I'd love to hear it.