Using HDMI and Mini DisplayPort as output devices for sound is supported only on Mac Pro (Mid 2010).
Mac Pro (Early 2009): No HDMI or Mini DisplayPort audio when using the AMD Radeon 5870 upgrade kit
You may notice that audio is not available through the Mini DisplayPort of an AMD Radeon 5870 graphics card installed in a Mac Pro (Early 2009). You may also notice this issue when you try to use HDMI as a sound output option.
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