Capture audio from youtube.
Capture music from video.
To capture audio, music or voice from a video, online or offline video (from Internet or from hard/optical drive), you can do by using the audio editing software, in this example I use Audacity.
Audacity is an audio editing tool/software, easy to use and good result. You can download at http://www.audacityteam.org/
Because the aim is capturing audio from the video, it is better to disable the microphone of the computer/laptop to avoid recording voice around you.
To disable microphone :
• Right click speaker icon at the tray icon.
• Click Recording devices.
• Right click at Microphone then choose Disable.
To start capturing audio from Video :
• Run the Audacity.
• Play/run the Video that you want to capture it's audio.
• Click the Record button to start capturing.
• If error or cannot start recording/capturing, try to switch the Audio-Host from MME to Windows WASAPI.
• To stop capturing by click the stop button.
• To save audio to general audio file, click File menu then choose Export Audio or using shortcut Ctrl+Shift+E, then choose the extension you want. For MP3 file you need to download the dll (lame_enc.dll). just follow the wizard.
This method is not only for capturing audio from video, also capturing audio from all sources are processing in the sound card, audio game, radio online, auxiliary input, etc.