FYI file encoding for speaker nicolas are 8bit unsigned integer whereas all other speakers are 16bit Signed int
sox -b 16 -e signed-int old_nicolas.wav new_nicolas.wav
does the trick
Nicolas also seems to be stereo (2 channels) instead of mono. This seems to solve both problems:
ffmpeg -i data/2_nicolas_4.wav -af 'pan=mono|c0=c1' nicolas_mono/2_nicolas_4.wav
FYI file encoding for speaker nicolas are 8bit unsigned integer whereas all other speakers are 16bit Signed int
sox -b 16 -e signed-int old_nicolas.wav new_nicolas.wav
does the trick