Silent render
My friend purhcased ACID 4.0 on my advise ( I own 3.0). The day he installed it he called me and began asking me a few "how to's." When I got to the part about rendering tracks, he kept trying without success. He ended up with the silent rendered track too. We tried for several hours to figure it out without success.
Then I suggested changing the buss setting to master and then it worked. The track render under the master buss.
If your using a MOTU audio card like he is maybe that's the ticket,but I suspect it's a bug if SF Acid 4.0. I'm going to wait until they fix the bugs before I upgrade. I fell kind of bad that I suggested the purchase but he's not too bothered by it.
Just my .$02.