Find a piece of video media from which you'd like to extract the sound. This could be a DVD, home video, or movie you've downloaded from the Internet. In the case of a DVD or home video, you'll first need to copy the video file to your computer. This can be accomplished with a video transcoding program such as HandBrake. The process is relatively simple, and usually just involves playing a DVD on your computer and pressing a "record" button with the transcoding program.
Download an audio conversion program, such as Switch Audio Converter Software from NCH Software. This program, like HandBrake, is available free of charge. Familiarize yourself with the program's interface, which in the case of Switch, is very easy to use.
After launching Switch, click the "Add File(s)" button located in the upper left corner of the window. This will bring up a dialogue box in which you'll be prompted to browse through the contents of your hard drive, and find the video file with which you wish to work. Select it and click "OK." If you wish to extract the audio from several videos at once, simply repeat this process. The files you choose will be added to the list of files to convert.
Decide what audio format you'd like your outputted files to be in. Switch allows for a variety of file formats, including MP3, AIFF, WAV, FLAC, OGG, RAW, GSM and more. You'll usually choose AIFF or WAV if you want to maintain optimal sound quality. Choose MP3 if you want to keep files sizes small.
Click "Encoding Options" to change the settings of your conversion. You can change the bit rate of an MP3 file, for example. Higher bit rates will sound better, but the resulting files will use more space. You can also choose whether you'd like to have mono or stereo audio. Click "OK." Click the "Convert" button in the lower right corner. A new audio file will appear on your computer.