When you master audio, you apply EQ and dynamic compression to your material to make it sound as good as possible. You can also normalize your material — that is, increase the gain to its maximum level — during the mastering stage. Doing this not only increases the volume of the audio, but it also improves the sound. 
For a few tips on normalizing audio, keep reading.

The EQ and compressors each have a gain controller which can be used to normalize audio. 

• Find the loudest part of your file and check to see how much space is left before the level peaks reach the 0 db limit. Check to make sure peak measuring is enabled so that you can see how much space is left between the peak limits and the 0 db limit. 
• Use the gain controller to directly "normalize" the signal and bring it as close to the 0 db limit as possible without going over it. 
  

Here are the easiest ways to find the highest threshold value in the file:

• Go to "Tools" > "Statistics" to see the maximum sample position.
• Go to "Tools" > "Find". Under "Name", select "Find largest peak" (maximum value).

Here's how to apply the normalization to the entire file:

• When you are previewing a volume setting, if the peak measurement shows -3.2 db as the highest value, you can increase the output volume in the Volume dialog window by 3.2 db. 
• After increasing the output volume, if the highest value is 0.0 db, click the "Select" button and choose "Select All" under "Select".
• Then, click "OK" to apply the settings to the entire audio file (instead of just the selected range).
• If you are happy with these settings, go to "Tools" > "Find" > "Name" > "Clipping" to check if the volume has gone into the red.