FFmpeg loudnorm filter - dual pass loudness normalization example - http://k.ylo.ph/2016/04/04/loudnorm.html

loundorm.py

#!/usr/bin/env python

import json
import os
from subprocess import check_call, check_output, STDOUT
import sys

if len(sys.argv) != 3:
    print "Usage: %s input.wav output.wav" % __file__
    sys.exit(1)

infile = sys.argv[1]
outfile = sys.argv[2]

TARGET_I = "-16"
TARGET_TP = "-1.5"
TARGET_LRA = "11"
SAMPLE_RATE = "44100"
BITRATE = "128k"

ffArgs = [
    "ffmpeg",
    "-hide_banner",
    "-i",
    infile,
    "-af",
    "loudnorm=I=%s:TP=%s:LRA=%s:print_format=json" % (TARGET_I, TARGET_TP, TARGET_LRA),
    "-f",
    "null",
    "-"
]

output = check_output(ffArgs, stderr=STDOUT)
outputJson = ''.join(output.splitlines()[-12:])
measured = json.loads(outputJson)

loudnormString = (
    "loudnorm=I=%s:TP=%s:LRA=%s:measured_I=%s:measured_TP=%s:measured_LRA=%s:measured_thresh=%s:offset=%s:linear=true:print_format=summary" %
    (TARGET_I, TARGET_TP, TARGET_LRA, measured['input_i'], measured['input_tp'], measured['input_lra'], measured['input_thresh'], measured['target_offset'])
)

ffArgs = [
    "ffmpeg",
    "-y",
    "-hide_banner",
    "-i",
    infile,
    "-af",
    loudnormString,
    "-ar",
    SAMPLE_RATE,
    "-b:a",
    BITRATE,
    "-write_xing",
    "0",
    outfile    
]

check_call(ffArgs)