ok, sam sobie odpowiem - a komuś może się przyda

c:>ffmpeg.exe -i "p2 - 128kbps.mp3" -codec:a pcms16le -ar 8000 p2.wav
ffmpeg version N-69258-gf994000 Copyright (c) 2000-2015 the FFmpeg developers
built on Jan 25 2015 22:14:27 with gcc 4.9.2 (GCC)
configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
le-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-lib
modplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinge
r --enable-libsoxr --enable-libspeex --enable-libtheora