This calls for ffmpeg.
C:\enc>ffmpeg -acodec pcm_s16le -ar 44100 -i "input.mp3" outfile.wav Input #0, mp3, from 'input.mp3': Duration: 00:03:52.75, start: 0.000000, bitrate: 255 kb/s Stream #0.0: Audio: mp3, 44100 Hz, stereo, s16, 256 kb/s Output #0, wav, to 'outfile.wav': Stream #0.0: Audio: pcm_s16le, 44100 Hz, stereo, s16, 1411 kb/s
Of course it also takes CCITT A-Law, CCITT u-Law and almost anything .wav as input.
To downmix to mono, use -ac 1
(as in audio channels=1).