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