import os
dst="Path\sourcefolder"
os.chdir(dst)
os.system("ffmpeg -i test.m4v test.wav")
os.remove("test.m4v")

for i in range(0,len(m4v_Paths))
    infile=m4v_Paths[i]  # example i th file "Path\source\folder\movie.m4v"
    outfile=os.path.join(os.path.splitext(m4v_Path[i])[0]+'.wav')  # remove '.m4v' extension add ' .wav'
    os.system("ffmpeg -i infile outfile")
    os.remove(infile)