preview playback, and rendering, are blocked if the highest layer with content has audio muted.
If the audio is muted on the highest layer with content at the current marker position, clicking on the play button in the preview window will change the play button into a pause button, but playing will be paused. This includes the case where there is just a single layer.
Similarly, having audio muted in the highest layer leads to an error right at the start of rendering.
If a lower layer has audio muted, neither of these problems occurs.
Muting the video for a layer doesn't have this effect, only the audio.
(I encountered this behavior with org.pitivi.Pitivi//master as well as with org.pitivi.Pitivi//stable)