For grins I decided to give ffmpeg.exe a try (from both websites linked above.)
Syntax is straightforward enough--something like this:
C:\>ffmpeg -i FILENAME.M4A FILENAME.WAV

Neil used Apple's lossless ALAC codec inside his M4A container, so as you'd expect with any lossless codec
ffmpeg's output WAV is identical to the one produced by mplayer in my tests.
I presume it's also possible to put another (lossy) codec inside an M4A container.
In that case ffmpeg & mplayer might not produce bit-for-bit identical outputs, although one hopes they would at least sound the same.

Originally posted by: FanFiltration
I don't think I would have a use for the converter after this one project.

Even if you don't do any more M4A conversions, here's another reason you might like to keep mplayer around:
"mplayer -speed 0.96" will play PAL movies at 24fps