I looked at the online manual and didn't see anything about Sound Injection. What page is it on?
To select an audio source push the applicable button on the remote. If you're watching TV, then hit tuner on your remote and you can then listen to music while watching TV.
I have a 7002 and will be trying to configure things as simple as possible so I can watch TV (mute) and play CDs at the same time. The way I have things currently configured, the only thing I can do is watch TV (mute) and listen to the radio. It isn't complicated; I just haven't taken the time to do it.
I hope I've helped you in some way. Then again, maybe I misunderstood your question.
A squirrel has been eating my tomatoes, so I took my AK-47 and shot him with an armor-piercing bullet. Unfortunately, it also went through my neighbor's car and his neighbors SUV before being stopped by a propane truck filling his neighbors tank. Half of the neighborhood is now gone, but so is that damn squirrel.