Hello all,
We are currently trying out Balena-sound for a DIY boombox project. We use a Raspberry Pi 4, an I2S module (UDA1334, that should be compatible with the I2S audio bonnet from Adafruit I think) and the 7" Raspberry Pi touchscreen. The goal is to display a GUI menu on the screen and use the I2S module to produce some sound.
Also interesting is the Balena-dash kiosk container. This opens the doors to load any webpage e.g. web radio, YouTube, streaming service, etc. The possibilities seem endless. 
Preparing to do some experimentation we created a Balena project, new git repository and put the Balena-sound and Balena-dash projects side by side with git subtrees.
In the project root we have a docker-compose.yml that looks like this:
version: '2'
volumes:
settings:
snapcast:
services:
boombox_main:
build: ./boombox-app
ports:
- "5000:5000"
# browser:
# image: balenablocks/browser:raspberrypi4-64
# privileged: true
# network_mode: host
kiosk:
restart: always
build: ./balena-dash/kiosk
privileged: true
network_mode: host
ports:
- "8080:8080"
volumes:
- "settings:/data"
# scheduler:
# restart: always
# build: ./balena-dash/scheduler
# privileged: true
audio:
build: ./balena-sound/core/audio
privileged: true
labels:
io.balena.features.dbus: 1
ports:
- "4317:4317"
volumes:
- "snapcast:/var/cache/snapcast"
sound-supervisor:
build: ./balena-sound/core/sound-supervisor
network_mode: host
ports:
- "3000:3000"
labels:
io.balena.features.supervisor-api: 1
Next we tried to create a new audio plugin according to the instructions on: https://sound.balenalabs.io/contributing/architecture/ (Plugin chapter at the bottom). The browser in the Kiosk container does not have a way to send audio to the Audio container via pulse audio, so we need the ALSA bridge I assume.
So we modified the Dockerfile of Kiosk:
# build tohora from source
FROM balenalib/raspberrypi3-golang as builder
RUN go get -d -v github.com/mozz100/tohora/...
WORKDIR /go/src/github.com/mozz100/tohora
RUN go build
FROM balenablocks/browser:%%BALENA_MACHINE_NAME%%
RUN install_packages cec-utils
COPY --from=builder /go/src/github.com/mozz100/tohora /home/chromium/tohora
COPY launch.sh /home/chromium/
# adding ALSA bridge
# https://sound.balenalabs.io/contributing/architecture/
ENV PULSE_SERVER=tcp:localhost:4317
ENV AUDIO_OUTPUT=balena-sound.input
#debian-setup.sh is a local copy of https://raw.githubusercontent.com/balenablocks/audio/master/scripts/alsa-bridge/debian-setup.sh
COPY debian-setup.sh /home/chromium
RUN /home/chromium/debian-setup.sh
#RUN curl -sL https://raw.githubusercontent.com/balenablocks/audio/master/scripts/alsa-bridge/debian-setup.sh | sh
RUN apt-get install alsa-utils
RUN apt-get install pulseaudio
RUN apt-get install pulseaudio-utils
RUN pulseaudio -D --system
# replace command that runs in start script
COPY alt_start.sh /usr/src/app/
RUN sed -i '$d' /usr/src/app/start.sh
RUN echo ". alt_start.sh" >> /usr/src/app/start.sh
If we then run the pactl list sinks command, in the Kiosk container, we can see the balena-sound.input sink.
But listing the ALSA devices with aplay -L does not reveal any Balena devices.
We where hoping that there was an ALSA device that we then could provide to the FLAGS (https://github.com/balenablocks/browser) environment variable that routes the audio from the browser to the balena-sound.input so the I2S module plays the sound.
However the sound from the browser is player on the audio jack of the Pi and not the I2S module.
It could also be that I have missed something in the Balena-sound configuration, but the only thing needed is the bellow ‘Custom configuration variable’ in ‘Device configuration’ right?
We’ve been messing around with with the Kiosk container, but can’t seem to get it to work.
Any help is appreciated.
Kind regards,
Woody
And some further documentation here: 
)