When you tick the Start time or End time option, the Secs box to the right of the option will become active.

Use the spin buttons to specify the point within the sound file to start or end playing from.

In the example above, the only 13.5 seconds of the sound file will be played, starting from 40.5 seconds into the file. Note, Opus does not wait for 40.5 seconds before playing the sound.