That's very weird. If you have checked everything (all your connections) and your DD receiver is set properly, I would assume that your VOOM stb receiver does not work in sending the DD5.1 audio signal. I will puy a service call to get a replacement but I will make sure that everything is connected properly according to your manual.
Just for the heck of it:
Are you hearing surround sound on all 5.1 speakers or just the front speakers? I will also check your manual to see if there is any difference between your DVD connection and a satellite receiver. There are two output digital output on the stb -- coax and toslink. Did you try both?