Simply go to the Convert tab, select the video file and choose MPEG Audio Layer 3 (MP3) from the drop-down menu. In the "Choose Quality" dialog you can choose the quality of the mp3 output file. Keep in mind that a higher quality implies a larger file size. You can modify the volume or select only a certain part of the video from the advanced conversion options. Click "Convert Video" and follow the progress in the activity tab.
