Well, they can't hear a knife.
3 seconds sound clip from the The Big Trail (1930) movie soundboard.
You can hear this line at 01:45:44 in the Blu-ray version of the movie.
Quote context
[...]
- Well... Well, he's turned up again.
- Yeah, well, he'll down the both of us.
- You've got to get him tonight.
- But they can hear a shot at night.
- Well, they can't hear a knife.
- They all know this knife of mine.
- Well, here's a knife they don't know.
- No, no, no, no. I afraid of that knife. I know where you got it. It will get us in trouble sure.
- How?
- Because a dead man's knife is bad medicine!
[...]