You seem to forget that we all heard the harpsichord playing.

6 seconds sound clip from the The Pit and the Pendulum (1961) movie soundboard.

You can hear this line at 00:37:08 in the Blu-ray version of the movie.

Quote context

[...]

- Your mother was walled up in her tomb while yet alive.

- Oh, no!

- From that day forth, the very thought of premature interment was enough to drive your brother into convulsions of horror.

- Nicholas broods about it constantly.

- Already he has told me he has heard Elizabeth walking the corridors, even that he has heard her speaking his name.

- You seem to forget that we all heard the harpsichord playing.

- No, I have not forgotten it, sir.

- It is why I said before that I thought that no one but Nicholas and myself knew about this.

- What do you mean?

- Someone, I fear, has discovered Nicholas' secret and is using it as a terrible advantage. A servant, perhaps.

[...]