Sound output through HDMI

The Mini DisplayPort to HDMI doesn't carry audio unless you have the Mid 2010 model 7,1 and there're no audio plugs on most TVs to work with HDMI since it's expecting audio with the HDMI. But check your TV manual to see if it has audio plugs that work with one of the HDMI ports.

If you have the Late 2008 model 5,1 Aluminum Unibody or Late 2009 model 6,1 White Unibody with the Mini DisplayPort rather than the Mini-DVI there is an adapter that uses the Mini DisplayPort and a USB port to combine video and audio to HDMI. It's only 2 channel stereo not 5.1 Dolby Surround.
http://www.monoprice.com/products/product.asp?c_id=104&cp_id=10428&cs_id=1042802 &p_id=5969&seq=1&format=2

From the Knowledge Base tab on the Monoprice website:

Answer: Using "Sound" Settings under System Preferences, or using "Audio MIDI Setup" - select the "USB Audio Device" as the audio output. This will switch the audio out from the built in speakers to the adapter.

You have the display set in Extended Desktop mode with the MacBook as the main screen. The Menu Bar and Dock will be on the MacBook display and your background screen on the TV but you can move the cursor onto the TV screen.

In System Preferences>Display on the MacBook screen there should be an Arrangement tab when you have the MacBook hooked up to the TV and both screens working. When you click the Arrangement tab do you see two monitors side by side? One of them will have a Menu Bar at the top. Just click on the Menu Bar and drag it to the second monitor. That will make the second monitor your main screen.

The TV will now have the Dock and Menu Bar and windows will open on it. But you can move the cursor to the MacBook screen and move windows from the TV over to the MacBook screen.

You can now use your MacBook in Clamshell Mode with a wired or Bluetooth keyboard and mouse. http://support.apple.com/kb/HT3131 When you disconnect from the TV your Menu Bar will automatically change back to the MacBook.

Or if you want to work on the MacBook screen while showing it on a TV you can check the Mirror Display box on the lower left hand side of the Arrangement tab under the two monitors box. That will give you the same screen at the same resolution on both the MacBook and the external monitor.