DisplayPort to HDMI Sound is actually set up by using the MIDI setup utility found in :
Applications > Utilities > Audio MIDI Setup.
If you purchased an iWires cable direct from Apple the instructions are in the printed booklet that comes in the packaging. Most people don't read this as it's normally the case that when you plug a cable in to anything it just works (VCR, TiVo, Xbox, video camera etc). In this instance Apple are failing to live up to their catch-phrase as final setup is NOT intuitive; "it just DOESN'T work".
The booklet instructions basically say do the following,
System Preferences > Displays. There are 3 tabs in the window; Displays, Arrangement and Color. Be sure you are on the Displays tab. Bottom left of the window tick "Show displays in menu bar" and then bottom right click "Detect Displays".
Now click on the tab named Arrangement. Bottom left of the window tick "Mirror Displays".
You should now only see 2 tabs, Display and Color. Be sure you are on the Displays tab. Select the desired resolution for your TV (probably 1080i). leave all other settings at their default values.
System Preferences > Sound. There are 3 tabs in the window; Sound Effects, Output and Input. Be sure you are on the Sound Effects tab. Middle of the window select your TV where it says "Play sound effects through:".
Select the middle tab named Output.
Select your TV from the drop-down list next to HDMI, below where it says "Select a device for sound output".
Using either Finder or Launchpad go to Applications > Utilities > Audio MIDI Setup.
On the left-hand side of the window you should now see HDMI. In the middle of the window, on the Output tab, where it says "Source:" select your TV from the drop-down list.
This last action below is NOT described in the iWires notes but was required in my case, it may also be required in yours. If you CANNOT see a speaker icon next to HDMI in the list on the left, do the following:
Bottom left of the window select the wheel icon with the down-pointing triangle. Make sure that sound output is selected and that you can now see a speaker icon next to HDMI in the list of devices.
If you still cannot hear sound, log out of your account and then log back in.
You will now have sound coming out of your TV from your computer
You may have to do some or all of the above to get sound out of your TV from your computer using the DisplayPort to HDMI cable. On my Aug 2011 MacbookPro I had to do ALL of the above. If you take your laptop to a different TV you will have to do it all again so be warned.
Hope this helps.