These are some tips you can use if the computer does not respond to spoken commands (Speech Recognition).
1. Check your microphone
Be sure that your microphone works, either with the Sound preference pane or an application capable of recording audio (such as iMovie).
2. How are you speaking?
Make sure you say commands in a calm voice that is not too soft or too loud. Watch the meter in the feedback window. If it shows only blue, you are too quiet. If it is red, you are too loud. When you say the command, do not shout or hesitate.
3. Check your environment
Avoid background noise. If you use Speech Recognition in a noisy environment, you may need to purchase a special microphone.
If background noise is unpredictable, try pressing the listening key 1/2 second before saying a command.
If you move your computer to a new environment, press the listening key 5 to 8 seconds before saying a command. You may need to adapt Speech Recognition to your environment again. If you use continuous listening while working in a noisy environment or having a conversation, Speech Recognition may become slow or less accurate. If this happens, turn off continuous listening.
4. Check the listening name
If you use continuous listening and the listening name is not configured as optional, make sure that the listening name is not blank. This setting may be found in the Speech preference's Listening tab. Alternatively, set Name Is: to "Optional before commands".
5. Mac OS X 10.4 or later only: Check for (null) Address Book entries.
Any contacts named "(null)" could lead to this issue. See this article for more information.
Some of this information may also be found in Mac Help, which is accessible from the Help menu on your computer. Speech Recognition does not work on all computers. See technical document 88229, "Mac OS X: Speech Recognition Does Not Work on Some Computers".