You may need to open Sound Settings and tell it to use the headphones or lineout (depending on which you used). An easy way to get to the Sound Settings is clicking on the volume indicator (top of the screen on the right hand side). In sounds settings there will be a box named "Play sound through" that lists possible output methods.
Other possible problems include:
- TV volume is muted or very low
- Netbook volume is muted or very low
- There is more than one audio in port and the one you chose does not go with the VGA video input (often TV makers will use visual indicators to show you which ports go with each other, on my TV there are boxes around ports that work together).
One thing to try is plugging in headphones. If the headphones work, but the TV doesn't, then the problem is likely on the TV side.