Hello,
is there a way to automatically completely silence those parts in the audio track that are below a certain volume? The background is that it is a voice-over track where the original sound is heard in the background, and it should be silent.

Since I am quite new to KDEnlive, it would also be of great help which layout I should work with. The audio track is only very roughly resolved.

black-video wrote:is there a way to automatically completely silence those parts in the audio track that are below a certain volume?

Yep. With noisegate, or gate plugin set as hard gate (which a noisegate izz ).
Although that what you ask for is an audio trick and not a video issue.

Hard gate =
Set max gain reduction at -50 or -60 or whatever (very low!)
Set ratio at max (straight down cut).
Then set the treshhold dB at wish (aka set at "certain volume" to silence below that ... your question).

To show you visual what happens then (this is set as you can see at cutting everything below -25dB)
click for big

In a plugin without GUI the settings/numbers can be used the same.