If Media Monkey no longer plays your MP4 files then this plugin will solve your problems. This plugin decodes all your .MP4 and your .M4A files as well as any AAC, PS AAC and the newer version HE ACC files.

The MP4 plugin also includes MP4 tagging and works well on Nero 6 and others. as it is a replacement for the in_mp4.You install this plugin simply by taking the zip file and  uncompressing the file and taking the  in_mp4.dll from archive and copying it into the  Winamp \ Plugins directory.

MP4 Plugin for Winamp