I can get picture from my desktop and videos on my television (using the mirror display) with a HDMI to mini adapter on my MacBook Pro but I get no sound. What can I do to get sound?
Audio output via HDMI is only supported for models from 2010 and later. You can check it's support by opening the "System Information" app from Applications > Utilities. Click on audio and see if "HDMI/Displayport Output" is listed.
If it is listed alt (option) click the Sound icon in the top bar and your TV should be listed and selectable in the output list.
Last but not least your TV needs to support audio via HDMI too.