FFMGPEG is available for Windows, is able to support variable bit rate and works really well for audio as well as video.
On a single file:
ffmpeg -i yourfilename.m4a -codec:v copy -codec:a libmp3lame -q:a 2 yourfilename.mp3
For batch conversion, with windows command line extensions:
FOR I IN (*.m4a) DO ffmpeg -i %I -codec:v copy -codec:a libmp3lame -q:a 2 %~dpnI.mp3
If you wish to specify an output directory rather than putting files where the originals were you can replace
\path\to\ouput\dir\ must be an existing directory.
Of course you can do a lot more with FFMPEG such as converting video formats, cutting and splicing video, extracting or adding soundtracks.
- Price: Gratis
- Open Source
- Cross Platform available for Windows, OS-X & Linux