After a ‘video only’ start, the DisplayPorts on most Apple machines have been carrying audio too since 2010 (check compatibility with your individual model).
“Apple Mini-DisplayPort video outputs are now enabled for AUDIO on the latest 2010 Macbooks (i5 / i7) and latest 2009-10 iMac 27″ and 21.5″ models (Older Macs with Mini DisplayPort (eg: 2008-09 MacBooks and iMac 20″ / 24 can NOT output Audio over Mini DisplayPort). Mini DisplayPort (MALE) 1.1a input and HDMI (FEMALE) 1.3b output – Supports 1920 x 1200 – 1080p FULL HD – Deep Colour – x.v.Colour – HDTV Advanced Features eg. 200hz / 600hz etc. Supports uncompressed audio such as LPCM.
This adapter is also compatible with PCs and other equipment that have Mini DisplayPort as well as Apple Macs. Supports HDMI 225MHz/2.25 Gbps per channel (6.75Gbps all channel) bandwidth – Powered from Mini DisplayPort source – Approx 15cm Long”