Listen to this original audio file, deliberately recorded with 'an initial quiet word' so you can Adjust the file levels...

Original Audio File

• Right-click the link above and from the menu that appears select Save As... or Save Link As...
  Save the file on your memory stick or some other appropriate place.
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• Open the saved file in Audacity it will look something like this...
  
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• Play the file and note carefully where the 'quiet voice' is located
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• Make sure that you click the Selection Tool...
  
• Select the section of the track (see basic Editing & Trimming if you are not sure how to do this) that needs to be changed...
  
• Use the Effect Menu and select Amplify...
  
  The Amplify Dialog Box opens...
  

Note: It is the Amplification (dB): Value that you need to change; this is a measure in decibels - A unit used to express relative difference in intensity, between two acoustic signals. When using Audacity, the difference between the current selected sound and the sound that will be produced.

If you set the value in the Amplification (dB): pane to be a...

• 0 (zero) then the selected wave will not be changed at all; No Change
• + value then the selected wave will become louder; Louder (the bigger the value the louder the end product)
• - value then the selected wave will become quieter; Quieter (the bigger the value the quieter the end product)
  Try it out remember you can undo any changes you make using the Edit Menu or the keyboard shortcut Ctrl+Z

After adjustment, the end product should sound something like this...

After Adjustment

and the wave look something like this...