RetroPie BGM v1.01

import os
import time
import random
#import pygame # if you don't have pygame: sudo apt-get install python-pygame
#also that line is commented out as we import the mixer specifically a bit further down.

#CONFIG SECTION
startdelay = 0 # Value (in seconds) to delay audio start.  If you have a splash screen with audio and the script is playing music over the top of it, increase this value to delay the script from starting.
musicdir = '/home/pi/RetroPie/roms/music'
maxvolume = 0.75
volumefadespeed = 0.02
restart = True # If true, this will cause the script to fade the music out and -stop- the song rather than pause it.

#local variables
bgm = [mp3 for mp3 in os.listdir(musicdir) if mp3[-4:] == ".mp3" or mp3[-4:] == ".ogg"] # Find everything that's .mp3 or .ogg
lastsong = -1
currentsong = -1
from pygame import mixer # import PyGame's music mixer
mixer.init() # Prep that bad boy up.
random.seed()
volume = maxvolume # Store this for later use to handle fading out.

#TODO: Fill in all of the current RetroPie Emulator process names in this list.
emulatornames = ["retroarch","ags","uae4all2","uae4arm","capricerpi","linapple","hatari","stella","atari800","xroar","vice","daphne","reicast","pifba","osmose","gpsp","jzintv","basiliskll","mame","advmame","dgen","openmsx","mupen64plus","gngeo","dosbox","ppsspp","simcoupe","scummvm","snes9x","pisnes","frotz","fbzx","fuse","gemrb","cgenesis","zdoom","eduke32","lincity","love","kodi","alephone","micropolis","openbor","openttd","opentyrian","cannonball","tyrquake","ioquake3","residualvm","xrick","sdlpop","uqm","stratagus","wolf4sdl","solarus"]

#test: Ran into some issues with script crashing on a cold boot, so we're camping for emulationstation (if ES can start, so can we!)
esStarted = False
while not esStarted:
    time.sleep(1)
    pids = [pid for pid in os.listdir('/proc') if pid.isdigit()]
    for pid in pids:
        try:
            procname = open(os.path.join('/proc',pid,'comm'),'rb').read()
            if procname[:-1] == "emulationstatio": # Emulation Station's actual process name is apparently short 1 letter.
                esStarted=True
        except IOError:
            continue

#ES Should be going, see if we need to delay our start

if startdelay > 0:
    time.sleep(startdelay) # Delay audio start per config option above

#can we capture OMXPlayer here to automatically handle a long splash?
pids = [pid for pid in os.listdir('/proc') if pid.isdigit()]
for pid in pids:
    try:
        procname = open(os.path.join('/proc',pid,'comm'),'rb').read()
        if procname[:-1] == "omxplayer" or procname[:-1] == "omxplayer.bin": # Looking for a splash screen!
            while os.path.exists('/proc/'+pid):
                time.sleep(1) #OMXPlayer is running, sleep 1 to prevent the need for a splash.
    except IOError:
        continue

#This is where the magic happens.
while True:
    if not mixer.music.get_busy(): # We aren't currently playing any music
        while currentsong == lastsong and len(bgm) > 1: #If we have more than one BGM, choose a new one until we get one that isn't what we just played.
            currentsong = random.randint(0,len(bgm)-1)
        song = os.path.join(musicdir,bgm[currentsong])
        mixer.music.load(song)
        lastsong=currentsong
        mixer.music.set_volume(maxvolume) # Pygame sets this to 1.0 on new song; in case max volume -isnt- 1, set it to max volume.
        mixer.music.play()
        print "BGM Now Playing: " + song
    pids = [pid for pid in os.listdir('/proc') if pid.isdigit()]
    emulator = -1;
    for pid in pids:
        try:
            procname = open(os.path.join('/proc',pid,'comm'),'rb').read()
            if procname[:-1] in emulatornames:
                emulator = pid;
                #Turn down the music
                print "Emulator found! " + procname[:-1] + " Muting the music..."
                while volume > 0:
                    volume = volume - volumefadespeed
                    if volume < 0:
                        volume=0
                    mixer.music.set_volume(volume);
                    time.sleep(0.05)
                if restart:
                    mixer.music.stop() #we aren't going to resume the audio, so stop it outright.
                else:
                    mixer.music.pause() #we are going to resume, so pause it.
                print("Muted.  Monitoring emulator.")
                while os.path.exists("/proc/" + pid):
                    time.sleep(1); # Delay 1 second and check again.
                #Turn up the music
                print "Emulator finished, resuming audio..."
                if not restart:
                    mixer.music.unpause() #resume
                    while volume < maxvolume:
                        volume = volume + volumefadespeed;
                        if volume > maxvolume:
                            volume=maxvolume
                        mixer.music.set_volume(volume);
                        time.sleep(0.05)
                print "Restored."
                volume=maxvolume # ensures that the volume is manually set (if restart is True, volume would be at zero)

        except IOError: #proc has already terminated, ignore.
            continue

    time.sleep(1);
    #end of the main while loop

print "An error has occurred that has stopped Test1.py from executing." #theoretically you should never get this far.