Q: I have connected an HD TV through one of the Mini Display ports with an HDMI cable on a Mac Pro model 3,1 but I can't get any sound through the TV.   How can I solve this Problem?

What graphics card are you using?

Where did the cable/adapter come from?

Per;

http://support.apple.com/kb/HT4241?viewlocale=en_US

Your Mini does not support audio out on its mini display port video out. It also discusses adopters to get the audio in the HDMI cable.

Does the HDMI audio show in the Output pane of Sounds system preference?

Does the Mini DisplayPort to HDMI adapter cable say it supports audio?

Your graphics card is not officially supported on your Mac Pro, so may not provide audio.

There are adapters to add optical audio to HDMI, such as:

<http://www.monoprice.com/Product?c_id=104&cp_id=10428&cs_id=1042802&p_id=6331&se q=1&format=2>

http://www.startech.com/AV/Displayport-Converters/Mini-DisplayPort-to-HDMI-Adapt er-with-USB-Audio~MDP2HDMIUSBA

maybe I can help. IF the card supports audio pass-through, you must use the "Sound" control panel in system preferences to select the connected device for audio output. Check under System Profiler, under built in audio, should be HDMI/Displayport, right under "optical Audio in" . Older mac Pro's, like the 3,1...which I also have, won't pass audio out via the MDP (MiniDisplayPort) socket,no matter what kind of graphics card you have installed.  Hence the need for for an  adapter that connects to the optical audio out of your mac Pro, and adds audio to the HMDI output. . you're just going to to have to buy a new adapter that connects to the optical out of your Mac pro, and be done with it. period.  Your Mac Pro is trying to pass audio thru the HDMI cable, but it just isn't happening, which is why the sound in the computer cuts off when the cable is connected.

JB