Hi Gomfucius!
It seems that's not possible to control a movie's volume programmatically and, what I think is worst, the mediaplayer completely ignores if the device is in silence or not, so sound will be played anyway. What I've done is to have the same video with sound and without sound and, just after loading the video, I check if the device is in silence mode or not and, if yes, I load the silenced video or, if not, I load the video with sound. It's a bit annoying having to do this way, but it's the only way I've found...
Hope it helped!