Welcome to CD Freaks
It depends a lot of on the copyright of the video and what you intend using the audio for, in much the same way as copying audio from another audio file.
If you are looking to extract the sound track from a few videos and don't mind doing this track by track, my preferred utility is AviDemux, a very good audio and video muxing/demuxing encoding/decoding freeware utility which you can get from their website here. It supports a wide range of audio and video codecs also.
To extract the sound track in AviDemux:
- Launch AviDemux, click 'Open', select the video and open it.
- If it requests to Index it, click 'Yes' and wait a moment to process.
- If it gives a warning about frame accuracy, aspect ratio, etc. just accept it.
- Go into the Audio menu and select "Save..."
- Give a filename for the audio track, with '.wav' at the end, e.g. "Music video soundtrack.wav"
- Click 'Save' and wait a moment to complete.