This may be slightly off piste;
We had the same symptoms though in a slightly different setup. This happened to any laptop (Mac/Windows) used to play DVDs onto a large hall projector with the sound connected to the mixing desk, PA system. We had great music sound and quality though voices or just anyone talking were virtually silent. This was a major issue and had our PA installer back out to sort it out, which he did.
The problem was the 3.5mm jack to a single XLR adapter. XLR being a mono connection and trying to force a stereo signal down it, for obvious and stated reasons, this didn't work and caused the symptoms being described in this thread.
This was resolved by our installer through creating us a stereo 3.5 mm jack to 2 XLR, essentially connecting the stereo channels separately all the way back to the mixing desk whereby they were joined to a single channel, similar to a CD, IPod or other stereo channel.
Now back to your issue, I'm wondering if there is the possibility of a mono connection anywhere in-between the laptops and the IWB speakers? 3.5mm jack cable? We have had a few of these floating around, I think I've got rid of them all.
Our W7, WXP laptops or macbooks don't seem to have experienced this issue on the standard IWB speakers? Sorry if it is off target; but the symptoms sound exactly the same and it may relate to others who are looking at the symptoms.
[Didn't realise the thread was an old one, my bad...though my experience may help.]