Looking to control a Media center / player with a Arduino

Hello,
I am currently working on a show with audio and video triggered by arduino.
For the audio-only elements I use the Rogue RMP3 Shield ( rMP3 Playback Module – Rogue Robotics )
wich is very easy to use and works perfectly.
Now I need to be able to do the same thing with a media player for video with sound.
Basically the only thing I need to control is the output volume of the media. (motion detector triggered)

Do you know of any hardware simlar to the rmp3 shield but that reads video as well? or how to 'hack' a regular media center to control its volume?
Or, the last option I thought of but which seems a little extreme, would be to be able to control directly the input or output of the pre-amplifier on which the media player outputs the sound.

I am opened to any suggestions, to control volume of a media player, triggered with a motion sensor

Thank you very much

Thank you very much.
I am sorry if my post wasnt clear enough. ( im french :wink: )
Basically what I needed was to control ( sorry, adjust) volume of a media player such as a dvd player or a mutimedia harddrive.

Your solution with the digital potentiometer seems really good, because very simple. I will try to implement that.
Cutting an audio wire and re-solder it wont lose audio quality?
(I do not know the first thing about audio :S )

thanks again KE7GKP.