Why do we chant OM?

When you attend a yoga class, it is most likely that the class starts with OM chanting. If you attend a Hindu ritual ceremony, it always starts by reciting OM. In fact, all the hymns (shlokas) of the Vedas and the Upanishads start with OM. A natural question that comes to mind is – what is the significance of OM and why it is recited at the beginning (and the end) of all yogic practices. In one of the classes during the currently ongoing 21-day yoga programs, I presented my thoughts on the topic. Please listen to this audio clip [audio:http://yogawithsubhash.com/audio/om-significance.mp3] from that class and see why I believe OM to be such a significant symbol and sound. I wrote about OM and its meaning in a previous blog post as well.
Do you chant OM in your classes? I would love to hear your thoughts on the audio clip and your own views on OM.