Remento does not currently support adding more audio or video to a recorded story. However, you can "re-record" a story, which will duplicate the prompt and invite you or your storyteller to record again.
Navigate to the Stories page
Find and open the story you would like to re-record
Tap the Make Changes button
Tap the More button
Select Re-record story
Confirm you'd like to re-record the story
Check your email. The prompt will be resent and can be re-recorded
After re-recording, you can choose to delete the original recording