Champion,
I too had a similar problem at one time with m4a files. Additionally tagging with respect to replay gain can be affected by not having an appropriate filter.
DC-Bass worked for me. It can be obtained at
http://www.dsp-worx.de/?n=15.
Additionally information can be obtained from from the MC wiki pages (
http://wiki.jriver.com/index.php/Main_Page), this is the page you will be taken to if you choose Help=>Contents in MC. The page and section that will probably be of most use to you is
http://wiki.jriver.com/index.php/MP4_and_M4A_File_Support#Apple_Lossless_Audio_Codec_.28ALAC.29. Although the information appears to be correct, you will need to take the screen shots with a grain of salt as the help section is not necessarily up-to-date.
First, install the DC-Bass source filter. Then, set MC to use it in MC's settings. To get to where the settings are in MC you will need to go to Tools=>Options. Select "File Types" on the left. On the right, file types are listed by category. Go to the Audio file types section and select "AAC Audio (m4a,aac)". Then, in the lower portion of the window for "Playback Method" choose "JRiver audio engine (using DirectShow filter)". For the "Source Filter" select "DC-Bass Source".
I hope this helps.
