If you're using the comcast remote you'll have to remap the volume key control to the AUX button. If you have the silver 3 device remote that most comcast users have you can do this by following these steps:

Setting the AUX to control the RCA
1. Press the AUX button once
2. Press and hold the setup button until the AUX button flashes twice
3. Enter 1254 (the AUX should flash twice)
4. Press the Setup button once again to finish

Mapping the Volume control to the AUX button
1. Press the AUX button once
2. Press and hold the setup button until the AUX button flashes twice
3. Enter 993
4. Press the AUX button once again.

Now, try pressing the cable button to set it as the current device and hit the guide button to ensure it is working, now press the volume + or - or Mute to see if your RCA unit responds.

I found that the 1254 code works for controlling the volume perfectly, but not the power for the RCA unit. I just leave it on all the time.

Skorpyo