Traditionally this was handled by doorpi. However, that pi has too much stuff hanging off it, and was very out of date. There was an issue of a loud pop occurring before any sound played, this was fixed in later firmware updates. Rob has now dedicated a pi (original B) for the purpose and documents it thus:

Latest Raspbian minimal image from the Raspberry Pi website.

• Medium overclock
• Graphics mem reduced to 16M
• Audio output forced to 3.5mm
• Hostname set

sudo systemctl enable tmp.mount

sudo apt-get install mpg123
sudo apt-get install sox
sudo apt-get install python-pip
sudo pip install paho-mqtt

These lines added before exit:

su -c "mpg123 /home/pi/sounds/indyboot.mp3" pi &
su -c "/home/pi/respawn --syslog /home/pi/squawk.py" pi &

Various scripts to make things happen.

#!/usr/bin/env python
 
import paho.mqtt.client as mqtt
import subprocess
import os
import logging
import signal
import time
 
 
last_state = None
 
max_playtime  = 15
 
sounds_path = "/home/pi/sounds"
 
# runs a command and terminates it after a specified timeout
def call_with_timeout(command, timeout):
    logging.info('call_with_timeout(%r, %r)' % (command, timeout))
    class TimeoutException(Exception):
        pass
    def alrm_handler(signum, frame):
        raise TimeoutException()
    try:
        old_handler = signal.signal(signal.SIGALRM, alrm_handler)
        signal.alarm(timeout)
        p = subprocess.Popen(command)
        retcode = p.wait()
        logging.info('call_with_timeout: command exited with code %s' % (retcode))
    except TimeoutException:
        logging.info('call_with_timeout: command exceeded timeout, terminating...')
        p.terminate()
        retcode = p.wait()
    finally:
        signal.signal(signal.SIGALRM, old_handler)
    signal.alarm(0)
    return retcode
 
# waffle waffle
def speak(data, timeout=max_playtime):
    command = ['/home/pi/pico.sh', data]
    call_with_timeout(command, timeout=timeout)
 
def getfiles(path, exts=[".mp3", ".wav"]):
    allfiles = []
    for dirpath, dirnames, filenames in os.walk(path):
        for filename in filenames:
            base, ext = os.path.splitext(filename)
            if ext in exts:
                #allfiles.append(os.path.relpath(os.path.join(dirpath, filename), path))
                allfiles.append(os.path.join(dirpath, filename))
    return allfiles
 
# make some noise for the vengaboys
def play(filename, timeout=max_playtime):
    allfiles = getfiles(sounds_path)
    filename = os.path.join(sounds_path, filename)
 
    if filename.endswith('/'):
        # pick a random file from a directory
        candidates = []
        for f in allfiles:
            if f.startswith(filename):
                candidates.append(f)
        if len(candidates) == 0:
            raise Exception('No files matching %s' % (filename))
        filename = random.choice(candidates)
    else:
        # single file requested
        if filename not in allfiles:
            raise Exception('File %s not found' % (filename))
 
    base, ext = os.path.splitext(filename)
    if ext == '.mp3':
        command = ['mpg123', ' -q', filename]
        call_with_timeout(command, timeout=timeout)
    else:
        command = ['play', ' -q', filename]
        call_with_timeout(command, timeout=timeout)
 
def on_connect(client, userdata, flags, rc):
    client.subscribe("sound/g1/play")
    client.subscribe("sound/g1/speak")
 
def on_message(client, userdata, msg):
 
    # ignore retained (non-realtime) messages
    if msg.retain:
        return
 
    if msg.topic == 'sound/g1/play':
	play(msg.payload)
 
    if msg.topic == 'sound/g1/speak':
	play('dong.mp3')
        speak(msg.payload)
 
 
m = mqtt.Client()
m.on_connect = on_connect
m.on_message = on_message
m.connect("mqtt")
m.loop_forever()

#!/usr/bin/env python
#
# Tim Hawes <me@timhawes.com>
# April 2015
#
 
import argparse
import logging
import logging.handlers
import os
import signal
import subprocess
import sys
import time
 
process = None
hup_received = False
term_received = False
 
parser = argparse.ArgumentParser(description='Respawn an application.')
parser.add_argument('--name', type=str, dest='name', action='store')
parser.add_argument('--delay', type=int, dest='delay', action='store', default=1)
parser.add_argument('--min-backoff', type=int, dest='min_backoff', action='store', default=1)
parser.add_argument('--max-backoff', type=int, dest='max_backoff', action='store', default=60)
parser.add_argument('--reset-backoff-after', type=int, dest='backoff_reset_after', action='store', default=30)
parser.add_argument('--syslog', dest='syslog', action='store_true', default=False)
parser.add_argument('--debug', dest='debug', action='store_true', default=False)
parser.add_argument('command', nargs='*')
args = parser.parse_args()
 
def setup_logging(log_level=logging.INFO, syslog=True, stdout=False, ident=os.path.basename(sys.argv[0])):
    logger = logging.getLogger()
    logger.setLevel(log_level)
    if syslog:
        syslog_format_string = ident + "[%(process)d]: %(message)s"
        syslog_handler = logging.handlers.SysLogHandler(address="/dev/log", facility=logging.handlers.SysLogHandler.LOG_USER)
        syslog_handler.log_format_string = "<%d>%s"
        syslog_handler.setFormatter(logging.Formatter(fmt=syslog_format_string))
        syslog_handler.setLevel(log_level)
        logger.addHandler(syslog_handler)
    if stdout:
        stream_format_string = "%(asctime)s %(message)s"
        stream_handler = logging.StreamHandler(stream=sys.__stdout__)
        stream_handler.setFormatter(logging.Formatter(fmt=stream_format_string))
        stream_handler.setLevel(log_level)
        logger.addHandler(stream_handler)
 
def run():
    global process
 
    start_time = time.time()
    process = subprocess.Popen(args.command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
    while True:
        line = process.stdout.readline()
        if line == '':
            break
        logging.info("< " + line.rstrip())
    returncode = process.wait()
    runtime = time.time()-start_time
    process = None
    if returncode == 0:
        logging.info('exit=%d runtime=%.3f' % (returncode, runtime))
    else:
        logging.warning('exit=%d runtime=%.3f' % (returncode, runtime))
    return returncode, runtime
 
def hup_handler(signum, frame):
    global process
    global hup_received
 
    if process is not None:
        logging.warning("received SIGHUP, sending SIGTERM to process")
        hup_received = True
        process.send_signal(signal.SIGTERM)
    else:
        logging.warning("received SIGHUP, but no process running")
 
def term_handler(signum, frame):
    global process
    global term_received
 
    if process is not None:
        logging.warning("received SIGTERM, sending SIGTERM to process")
        term_received = True
        process.send_signal(signal.SIGTERM)
    else:
        logging.warning("received SIGTERM, but no process running")
        term_received = True
 
signal.signal(signal.SIGHUP, hup_handler)
signal.signal(signal.SIGTERM, term_handler)
 
ident = os.path.basename(sys.argv[0])
if args.name is not None:
    ident = ident + "/" + args.name
 
if args.debug:
    setup_logging(log_level=logging.DEBUG, stdout=True, syslog=False, ident=ident)
else:
    setup_logging(log_level=logging.INFO, stdout=False, syslog=True, ident=ident)
 
if args.delay > args.min_backoff:
    logging.debug('increasing min-backoff to match delay (%d)' % (args.delay))
    args.min_backoff = args.delay
if args.min_backoff > args.max_backoff:
    logging.debug('increasing max-backoff to match min-backoff (%d)' % (args.min_backoff))
    args.max_backoff = args.min_backoff
 
exit_requested = False
backoff = args.min_backoff
 
logging.info("command: %r" % (args.command))
 
while True:
    start_time = time.time()
    returncode, runtime = run()
    if term_received:
        logging.debug("exited after SIGTERM")
        break
    if hup_received:
        logging.debug("exited after SIGHUP, restarting immediately")
        hup_received = False
        continue
    if returncode == 0:
        if runtime > args.backoff_reset_after:
            backoff = args.min_backoff
            logging.debug('resetting backoff to %d' % (backoff))
        else:
            logging.debug('delaying for %d after a successful run' % (args.delay))
            time.sleep(args.delay)
    else:
        logging.info('backing-off for %d seconds' % (backoff))
        time.sleep(backoff)
        backoff = min(backoff*2, args.max_backoff)
        logging.debug('next backoff will be %d seconds' % (backoff))
 
logging.info('exiting respawn')

#!/bin/bash
pico2wave -l en-GB -w /var/tmp/pico.wav "$1"
play -q /tmp/pico.wav
rm -f /tmp/pico.wav

These live in /home/pi/sounds and can be wav or mp3. SCP new ones into here.

pi@squawk:~ $ ls -l /home/pi/sounds/
total 504
-rw-r--r-- 1 pi pi   9249 Oct  4 22:32 alert12.mp3
-rw-r--r-- 1 pi pi   8594 Oct  4 22:32 bingo.mp3
-rw-r--r-- 1 pi pi  59350 Oct  4 22:32 canttouchthis.mp3
-rw-r--r-- 1 pi pi  11703 Oct  4 22:32 cheese.mp3
-rw-r--r-- 1 pi pi  71457 Oct  4 22:32 commandcodesverified_ep.mp3
-rw-r--r-- 1 pi pi   7713 Oct  4 22:32 computerbeep_4.mp3
-rw-r--r-- 1 pi pi  21759 Oct  5 02:24 dong.mp3
-rw-r--r-- 1 pi pi 118725 Oct  5 01:48 doorbell.mp3
-rw-r--r-- 1 pi pi  44329 Oct  5 02:07 indyboot.mp3
-rw-r--r-- 1 pi pi  90825 Oct  4 22:32 scatman.mp3
-rw-r--r-- 1 pi pi   5486 Oct  4 22:32 touchdown.mp3
-rw-r--r-- 1 pi pi   6414 Oct  4 22:32 uhoh.mp3
-rw-r--r-- 1 pi pi  27584 Oct  4 22:32 whistle.mp3

mkdir ~/pico
cd ~/pico
wget  http://incrediblepbx.com/picotts-raspi.tar.gz
tar -zxf picotts-raspi.tar.gz
sudo cp -R usr /
cd /usr/src/pico_build
sudo dpkg -i libttspico-data_1.0+git20110131-2_all.deb
sudo dpkg -i libttspico0_1.0+git20110131-2_armhf.deb
sudo dpkg -i libttspico-utils_1.0+git20110131-2_armhf.deb
rm -rf ~/pico

Pi lives above door. Connected to shit amp board. Amp board and speaker being replaced very soon - will document this then.