What mood does the author's use of silence create as the husband and wife lie in bed waiting for their wish to come true?

Well, what could be more creepy than laying in silence, in the dark, listening to the creaking and settlings of an old house?  Add to that the fact that you think your previously dead son might be coming back from the grave...

"The old man, with an unspeakable sense of relief at the failure of the talisman, crept back to his bed, and a minute or two afterward the old woman came silently and apathetically beside him.

Neither spoke, but both lay silently listening to the ticking of the clock. A stair creaked, and a squeaky mouse scurried noisily through the wall. The darkness was oppressive, and after lying for some time screwing up his courage, the husband took the box of matches, and striking one, went downstairs for a candle."

The silence does indeed create a particular mood for the story.  The reader is literally waiting for something to happen.  This is symbolized by the way that the clock keeps on ticking.  It is the only thing "moving" in the dark room...or is it?  This trick of reducing the light and sound in the room focuses the readers attention and provides a basis for anticipation. In a literary sense, it is building suspense.

Also, look at the way the sound of the knocking comes through:

"at the same moment a knock, so quiet and stealthy as to be scarcely audible, sounded on the front door."

That's the first one, almost so quiet you wonder if it really happened.  How bad can the outcome be if the "thing" on the other side of the door knocks so gently?  But then:

"his breath suspended until the knock was repeated. Then he turned and fled swiftly back to his room, and closed the door behind him. A third knock sounded through the house."

The knocking becomes more frantic, more pressing.  The thing out there really wants to come in.  The building sound of the increased rapping testifies to that.

"A loud knock resounded through the house."

Now it is getting angrier.  It wants in.  It is hitting the door harder.

"A perfect fusillade of knocks reverberated through the house,"

Nothing friendly would pound like that.  Do you see how sound (or the absence thereof) can contribute to the creepy mood of the story?  Sure, the author could have thrown the door open and showed you what was outside, but would that have been more awful than the shuffling unknown that was standing outside, desperate to get in?