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 added to the Speaker group. Speakers can also be referenced in the Sessions library using a role that links to their profile within the session details of your Agenda items.
If you are not using a People group to identify your Speakers, or prefer to filter profiles into the Speakers section based on the "Speakers" role you have assigned in the Sessions library then you will need to update the section to display Specific Session Roles and select the Speakers option from the dropdown menu.