For some AMD Radeon graphics cards when using the Catalyst driver, the HDMI audio support isn't enabled unless using the simple DVI to HDMI adapter included with the graphics card itself... If you use another DVI-to-HDMI adapter, it won't work with Catalyst. AMD intentionally implemented checks within their closed-source driver to prevent other adapters from being used, even though they will work just fine...
October 8, 2013
AMD Intentionally Crippled Their HDMI Adapters
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