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:

https://projects.drogon.net/raspberry-pi/wiringpiface/#comment-2200

 

Cheers,

Drew