While the text in a caption file mostly contains speech, captions also include non-speech elements like sound effects that are critical to understanding what the video is playing.

Sonix does not allow blank paragraphs/subtitle so the first step is to enter "non-speech element" into an existing paragraph or subtitle. Once you have done that, press enter to break it off into its own paragraph/subtitle. From there you can then set the start/end time of the new subtitle.