Several things to check:
1. Set the radio to disable...no plugin selected in the config.
2. With streetdeck windowed...double check your windows volume levels. See if one is muted and adjust to see if it fixes.
3. Try from a fresh start, there have been issues with standby/resume and the radio.
4. DO NOT ADJUST fader, balance, or channels in streetdeck. This doesnt work on the T3 on the latest versions anymore. I only get front channel when in SD now.
5. You may need a different version of the via audio deck drivers. I used the ones that came off my SD DVD, but I think there is a post with links to them somewhere on this forum.
6. Do you have your cables connected between both head units properly. There is a T3 install guide on the forum. If you dont have the front and rear(labeled "real" on the cables on the harness) then you arent passing the sound from the radio/tv to the main unit properly. I messed this up the first time I hooked it up.
Let me know how it goes...