from subprocess import check_call

ok = check_call(['ffmpeg','-i','input.mp3','output.wav'])
if ok:
    with open('output.wav', 'rb') as f:
        wav_file = f.read()

note:

• http://superuser.com/questions/507386/why-would-i-choose-libav-over-ffmpeg-or-is-there-even-a-difference
• http://stackoverflow.com/questions/9477115/what-are-the-differences-and-similarities-between-ffmpeg-libav-and-avconv