What UBBERgoose said, however - assuming that your equipment has the normal connections - run a phono lead from the outputs of your Freeview box to a spare input on your surround system, and set the system to this input when you watch the Freeview box. If the Freeview box has a sound output and the surround system has a sound input (even if it's in use, unless it's optical) you should be able to do it, but more details would be helpful!