I'll send this up on your behalf with some notes to see if anything can be done. At this time, I don't have a solution to offer, except for using the separate soundbar remote to adjust the soundbar's volume.
I have a Samsung PN58C8000 Plasma TV and it is connected to a Samsung HW-C450 Soundbar. The TV's speaker is turned off. When I use the TV's remote to adjust the volume a message displays saying "Not Available". I read all the other posts on this issue and I understand that if I turn the TV speakers back on the problem goes away. I also understand that the TV is confused because I am pressing the volume button when its internal speaker is turned off. But COME ON SAMSUNG, this is a $2500 Television - it should sense that an external speaker is in use and not display this message when I use the remote volume control. In fact, this message is literally useless under any circumstances, so why keep it? The annoyance of the message overrides the usefullness of the message - PLEASE GIVE US A FIRMWARE UPDATE TO DISABLE IT. I do NOT want to have the TV speakers competing with the soundbar at the same time. I would ask my fellow CNET users to lend their support and would ask Samsung to listen. There are a lot of us out there with this problem. Again - pitiful for a $2500 TV.