YouTube-mp3.org is the easiest online service for converting videos to mp3. You don't need an account, the only thing you need is a YouTube URL.
They will start to convert the audio track of your video to mp3 as soon as you submit it and you can download it. Different from other services, the whole conversion process will be performed by their site and within a matter of minutes, you download the audio file from their servers.
Because conversion is performed online, everything is platform-independent: You can use it with your Mac, a Linux PC or even an iPhone. All conversions will be performed in high quality mode with a bitrate of at least 128 kBit/s. The service is completely free, please allow approximately three to four minutes per video.
Click this link to create MP3 files with http://www.youtube-mp3.org.