Yeah, sibilance can come from many places along the audio "journey".
Some speakers sound sibilant nearly all the time, in which case, it's probably the speakers themselves!
Some amplifiers can also produce sibilance as can processors.
The soundtrack itself can be sibilant or the microphone could have picked up some sibilance when the recording itself was being made!
One thing that hasn't been mentioned though is that soundtracks for commercial movie theaters are almost always mixed with a boost in the treble to make up for the long distance from the speakers to the audience and for all the human bodies that are sucking up acoustical energy. When the movie is re-engineered for DVD/Blu-ray release, they don't always re-mix the soundtrack. So if you get that original, "hot" mix at home, it often sounds sibilant or harsh. This is what THX Re-EQ was made for - it basically just rolls off the high end.
Most receivers, even if they aren't THX badged, offer some sort of "Cinema EQ"-type setting. For most movies these days, you want to leave that setting off because movies are being re-mixed in the studio specifically for home release. But for the odd movie that still has the original theatrical soundtrack with the hot treble, engaging Re-EQ can often bring the high end back down to where it should be and bring the sibilance under control.
