Gordon of WiringPi and drogon.net has created a nifty metronome demo program for the PiFace:


Here's the source code:


And the installation instructions:


As you'll notice in the above instructions, Gordon has figured out how to do PWM with the PiFace outputs!  Here is rather noisy application, middle A being "played" on the relays:


The background of Gordon's PWM PiFace work is in this comment on his blog: