There was some difficulty with using the technique I had used in April for playing an announcement file directly from Finder into the live broadcast. Going through the steps, found that the following sequence is required: First change system audio out to Jack device using Audio MIDI setup utility. Then the Jack setup file which I had created earlier can be loaded.
But in general, this is a bit risky - a better way to do announcement insertion would be to drag and drop the relevant files into the timeline on Reaper. This can be done even when Reaper is recording. The track into which the file is dropped should not be armed for recording - only then will the file play back if Reaper is recording on another track.