You can set MC to use DirectShow decoding for the m4a file type and convert m4a files. It is a good option if you want to convert some m4a files from time to time (perhaps for using them with a portable or burning MP3 CDs). Here are some instructions for configuring it:
http://yabb.jriver.com/interact/index.php?topic=55606.msg377942#msg377942Another option is to use dBpoweramp. I'd recommend it if the intention is to convert all old m4a files and use only the new converted files after that. The DirectShow decoding system has only a limited tag reading support. dBpa should be able to include all file tags.
You can also use iTunes and convert the files to temporary AIFF files. MC has native support for AIFF files and it can convert them to other formats. You can enable AIFF tagging support in MC in Plug-in Manager (Tools > Plug-in Manager > Input > Wave & AIFF plugin > Configure...). If you have enough space and you are comfortable with using iTunes, this might be a viable option. In any case the m4a files need to be decompressed in a way or another before they can be converted to another format (AIFF is an uncompressed format).
