when i want to watch movies which are avi files on my lcd tv using an hdmi cable, i am unable to get sound from the tv, so i am watching the film on my tv but the sound comes from my laptop. i know that i can get the sound from my tv as i have done in the past but i have no idea why i cant now. please can any one help

My Computer
System One
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- Manufacturer/Model
- sony vaio ar61m