The content that is displayed in your event app sections is determined by the filters that are selected. In the People section specifically, you can select between All People, Specific People Groups, or Specific Session Roles.
The default for the Speakers section of the event app is to display any People profiles that have been assigned the role of Speaker within a session. Speakers may also be referenced in the People library using a group to help identify them within the list of profiles, and for the purpose of sending targeted announcements.
If you are not using session roles, or prefer to filter profiles into the Speakers section based on the "Speakers" group you have assigned within the People library then you will need to update the section to display Specific People Groups and select the Speakers option from the dropdown menu.