One feature you may have overlooked with Apple’s revised mid 2010  MacBooks and MacBook Pros is that if you connect an HDMI adapter to the Mini DisplayPort, the HDMI signal now carries both video and audio.  To use this new feature, you must select HDMI in the Sound System Preference pane.

Previous models of MacBooks and MacBook Pros only support video out when using a Mini DisplayPort DHMI converter.  According to Apple, this new feature requires a “VESA-compliant adapter for full HDMI capabilities”, which includes all Apple adapters, and adapters from Griffin and Dr. Bott.