Support for SermonSpeaker 5.x is in bugfix mode only, the same as it is for Joomla 3.10.
I will not add any new features to this releases.
Please upgrade to SermonSpeaker 6.x and Joomla 4.x, which is better anyway.
When you create a new sermon, you should see a list of your files on Amazon S3. Select one, fill out the other information and save the sermon.
If you don't see the list of files, it means you have entered something wrong.