This was helpful for me to figure out how to get a random interval of silence into a music playlist, thank you.
To expand on your answer and to address the other part of Kim's question, if you want to have this silence play after each randomly selected track in the playlist, you can use nested groups within the Music Playlist editor.
For example, if you have two tracks you want to randomly select with a random length silence after them:
Random Continuous Group (Top parent) - Loop Count Infinite
Sequence Continuous Group (Weight 50, loop count 1):
Track 1 Segment
Randomized Silence Segment
Sequence Continuous Group (Weight 50, loop count 1):
Track 2 Segment
Randomized Silence Segment
You can add new groups in the right-most column of the Music Playlist Editor of WWISE 2019.2.13. I assume similar in other versions. Then you can use these groups hierarchically like in the above example to achieve the desired design.