Fix error when run ffmpeg on Windows

[nvh95]

On some Windows machine, when subprocess ffmpeg runs, leads to errors look like this.

bellowubprocess.CalledProcessError: Command ‘[‘ffmpeg’, ‘-y’, ‘-i’, ‘a.mp4’, ‘-ac’, ‘
1’, ‘-ar’, ‘16000’, ‘-loglevel’, ‘error’, ‘c:\users\boss\appdata\local\temp
\tmpbvht_u.wav’]’ returned non-zero exit status 1

Those errors can be resolved by add argument shell=True to subprocess.checkoutput

[agermanidis]

LGTM.