MPlayer ought to do it. A command such as:
C:\>mplayer -ao pcm FILENAME.M4A
should produce a file with default name audiodump.wav
Or FFmpeg. These two sites offer precompiled Windows binaries (ffmpeg.exe)
I don't know its syntax offhand, but "ffmpeg --help" or "ffmpeg -?" ought to get you started.
Both MPlayer & FFmpeg include the same library for decoding, called libavcodec.
I used mplayer to decode Neil S. Bulk's '77 Dolby Stereo capture before Belbucus released his.