You get this tab with App Menu → Manage → Find in Files
Here you can search for text in all your Audio Notetaker projects, including most of the text in any slides you have imported. You can restrict your search by topic or speaker, though this would not normally be necessary.
You can double click on any of the search results to open the Audio Notetaker Project at that section.
If you want to just search the text in an open Project then you can use Find.