Here's how you can use voice commands to search in Android. Note that you need to be running Android 2.1 or higher. There are three ways to access Voice Search:
1. Press and hold the Search button.
2. Touch the microphone icon in the Google search box.
3. Touch the microphone icon in the browser's URL box.
4. A menu will appear asking you to search the web or use Voice Actions.
5. You can tap the Help button if you aren't sure what Voice Actions are or how to use them.
For more, see the original article at the link below.
Using Voice Actions | Google