Try Goldwave Digital Audio Editor from
www.goldwave.com Of course you have to download the lame encoder too and put it in the goldwave folder.Load the mp3 file you want to convert, click on save as and choose your bitrate.The trial version can be used forever.
About real files I have no idea but you can try..Even though vob files are not listed in the compatible file list, I was able to load vob files and save the audio directly in wav and mp3 formats.A very versatile program that is worth having in the kitty for any audio enthusiast.
Out of context but may be useful for those interested: Recently I purchased a music DVD with 15 tracks. I wanted to extract the audio from the tracks and convert them to mP3. After trying various programs like XMPEG, flask MPEG etc., I finally stumbled upon the fact that vob files can be loaded into the Goldwave editor (notwithstanding it is not listed as compatible) and the audio can be saved in whichever format one wanted.It is very simple and I now use Goldwave exclusively for extracting audio from vob files.( I have only extracted music which is 2 channel-stereo- AC3, and have not tried any AC3 5.1 tracks)