Audacity is free, open source software for recording and editing sounds. It is available for Mac OS X, Microsoft Windows, GNU/Linux, and other operating systems.

This is not the absolute best program to work with, as the effects and quality are not top notch. However, it is free, and if you are looking to be just a casual sound manipulator, it will fit your needs.

Step 1: Open up Audacity

The icon looks like this

Step 2: Open the audio file

Click on File -> Open. Choose the audio file you want to modify.

Step 3: Edit the video

Highlight the section of the audio you want to edit out by clicking and dragging. (Your mouse will turn into a hand when you can click and drag.)

Step 4: Save the project

To save the file that you are working on, click File -> Save Project As. Be careful! This only saves the file that you are currently working on as an Audacity file. You have to follow the next to actually create an audio file that you can listen outside of Audacity.

Step 5: Export your audio file

Now, we'll turn the audio file into either .WAV or .MP3 format. While .WAV files have better quality, they are larger than .MP3's. For the purpose of creating a video with an audio file, .MP3's are suffiecient.

Step 6: Save the audio file

Choose the location to save your audio file, and click "Save."