Tap out any rhythm on this simple NXT Echo Telegraph, and the NXT will answer you by playing back your exact rhythm with the NXT's speaker, as if a distant friend on the other end of the line was answering and repeating you. The program demonstrates use of the File Access block to store the rhythm timing data in a file and then reads it back out to repeat your rhythm.
While writing the program for this project, I discovered a neat little NXT-G trick. You know how you can't wire a value into a Time Wait block in an attempt to specify how long to wait (because the Wait block has no data hub)? Well, instead of using a loop and a timer to do a value-based time wait, you can use a single Sound block to play a tone with the time value wired into the Duration, Volume set to zero, and Wait for Completion checked, thus producing a silent delay of the requested time with just one block.