It is common to ask students for "eyes and ears" before an announcement. Modify this to include "monitors off" so that students will focus on you during the announcement and not their computer. Don’t start your announcement until everyone has turned off their monitor. If possible, make announcements at a location in the classroom where you can see all of the students’ monitors.
Example: "I have an announcement. Please turn off your monitors."
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-lab-rules