Some suggestions:
Volume Levels - You need to work on dynamics. By this, I mean loud parts and sort parts. Not just in music, but in sound effects and dialogue too. For starters, the music needs to be slightly louder. I was having trouble hearing it. Music can be louder in terrain viewing scenes, and slowly fade in volume as any dialogue or sound effects come in. Music can increase in dynamics in fighting scenes to make things more dramatic. Another example is having footsteps growing in volume as the character comes from far away, then closer to the camera. This pushes the 3D feel of the cinematic.
Random Sounds - At the beginning I hear the child death sound repeated, swords attacking, etc. I don't think they are suppose to be there. If they are, you haven't explained the situation or atmosphere very well at all. Also heard a fire sound inside the castle near the start. Don't think that's suppose to be there. Set fire sounds to 0% using a trigger.
Sound Effects - There's basically no sound effects at all. A cinematic like this needs a LOT. When the father and son are in the tavern, consider adding a crowd talking sound, an occasional guy laughing out loud, a beer spilling over, etc. Add some laughs or growls or angry sounds to dialogue too. You could add a tree swaying sound when passing trees, footsteps, door openings. Sounds sure do make the cinematic realistic. Remember your volume dynamics. Don't play any one sound at 100% volume, unless your trying to shock the audience. Just Google sound effects. Lots of things will pop up, or even record your own.
Dialogue - I found some of the diagloue cheesy, or unsuiting to the character. Lots of cliche' lines, not much originality. One example of poor dialogue is the father and son conversation in the tavern near the beginning of the cinematic. Pretty sure the son must be 25-35 years old. He was saying things a little kid says to his daddy. He didn't sound like a grown man going to war. Add a more realistic personality to your characters.
Grammar - Not much to say. Improve your grammar within the dialogue.
Movement - Some scenes are boringly still. I noticed some scenes where units may patrol in the background, but try to do more. Maybe when units are having conversations, make the turn. Maybe if the footmen brings bad news, the commander turns his back on him and lowers his head. Make a "sigh" sound effect to play at that moment. Turn the commander slowly back towards the footmen and then make a reply. Just an example. Plenty of scenes need some work.
Making a quality cinematic on a WC3 engine by yourself is very time consuming and difficult. I never have been very into WC3 cinematics because most struggle to make them remotely interesting. If you properly implement the above points, you can start to turn a WC3 cinematic into a proper anime TV show or something. Bring this cinematic to life!
Good luck
Edit: Made a cinematic demonstrating sound dynamics and attached it. Triggering isn't hard, but I documented it anyway.