YouTube videos can be converted to mp3s online using a service such as Video2MP3, YouTubeMP3, or ListenToYouTube. Similar services can be found by searching for "YouTube to MP3" on Google.
Many websites rip the audio from YouTube videos in low quality, furthering the degradation of audio that has already been compressed. If you need higher quality audio, you can use services such as MP3Fiber or Vubey. These websites give you the choice to extract MP3s at a higher bit rate. While the higher quality MP3s will sound better, be aware that they will also take up more hard drive space.