Coincidential timing?

While this is probably so the audience can focus on Gandalf's speech, you could explain it by saying that the troll was attacked/otherwise distracted temporarily, and the timing was a coincidence.

On purpose

This was done on purpose. When Pippin says *I didn't think it would end this way*, you can still hear the troll pounding on the gate. For a few moments into their talk, you can hear the pounding very faintly. Then when Gandalf starts talking about the white shores, the pounding is purposely faded out so that you can focus on what Gandalf is saying (it's like tuning the noise out). It is also done so that when the focus suddenly goes back to the troll pounding on the door, it's like a *wake up* call (it startled me, anyway). I think this nitpick should be refuted.

Dramatic effect!!

Obviously, the camera pulling in close and the sounds of the troll pounding on the door and the men struggling to keep the door together fading away was done for dramatic effect. It was supposed to represent the fact that Gandalf was able to draw Pippin's thoughts away from the troll and the impending doom that is pounding on the door and bring him into a sense of calm. But when they bring their focus back to reality, the troll is still there pounding away.
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\rImagine a war movie and we're following a soldier character into battle. Suddenly, he is shot and everything goes absolutely silent even as the battle rages around him and we watch his lifeless body fall to the ground in super-slow motion. It is a valid nitpick to point out that we should have still heard gunfire and that it would never take ten seconds for someone to fall to the ground after being shot? No!\r

Though it has been stated that the troll's pounding was merely muted by Gandalf's soothing voice, perhaps the troll merely backed up for a moment to get some momentum before slamming into the door again. I mean do you think you could pound a door down with your fists, or even a big hammer? I'd have to back up and get a running start to shoulder rush it, and weaken it before taking it on with my fists or a weapon.