Using iMac in Home Cinema
According to Apple Support you can get MDP to HDMI adapters with sound support from Griffin and Moshi.


According to http://store.moshimonde.com/faq-minidisplayport-to-hdmi:


  
  Which audio formats are supported via Apple's Mini DisplayPort?
  
  
  Recent Apple/Mac models (see above table for compatibility list) do
  support multi-channel 24-bit digital audio output such as Dolby
  Digital Surround and DTS as well as 2-channel stereo. Our Mini
  DisplayPort to HDMI adapter is a translation device that would then
  pass the audio bit stream unaltered to your AV receiver or TV for
  further processing.



I don't see why that would be necessary.
As long as audio output is set to HDMI.