First, press Go. You'll hear "This is a Test" three times simultaneously: Once preceded by a silence of 0.1 seconds, then another preceded by a silence of 0.3 seconds, then a third preceded by a silence of 0.5 seconds. But that's not a very realistic echo. All the sounds are the same volume. Use the "Scale by" block to reduce the volume in the second and third echoes, by a scaling of 0.5 and 0.25.
When you press Go, you're executing all three "play samples" blocks at once. What's really going on is that those sounds are being "mixed." Try hitting the space bar to hear the same thing, but mixed all together. Now, try reducing the volume in those blocks.
You'll need the SamplesExtension to run this project.