To send an announcement in a course with multiple merged sections begin by clicking on the "Announcements" tool

Click on the blue "New Announcement" button and create your announcement. When ready scroll to the bottom of the page and click "Create and Attach" under "Additional Release Conditions"

Next, click on the "Select Condition Type" dropdown and click on "Section enrollment"

Next, click on the "Select Section" dropdown and then the section you want to send the announcement to, then click the blue "Create" button

Finally, scroll down and click on the blue "Publish" button to send to the section.