No sound on Raspberry pi

[hugootto]

Hi!

I installed Gradio 7.2 on my Raspi, freshly installed OS is:

pi@raspberrypi2:~ $ cat /etc/os-release
PRETTY_NAME="Raspbian GNU/Linux 9 (stretch)"
NAME="Raspbian GNU/Linux"
VERSION_ID="9"
VERSION="9 (stretch)"
ID=raspbian
ID_LIKE=debian

Gradio 7.2 was installed from https://flathub.org/apps/details/de.haeckerfelix.gradio
(de.haeckerfelix.gradio.flatpakref)

Expected Behavior

sound...

Current Behavior

no sound, error messages instead:

pi@raspberrypi2:~ $ flatpak run de.haeckerfelix.gradio
** Message: 12:15:26.105: gradio-app.vala:185: Gradio 7.2
** Message: 12:15:26.198: gradio-app.vala:193: Locale dir: /app/share/locale

(gradio:3): dbind-WARNING **: 12:15:27.329: Error retrieving accessibility bus address: org.freedesktop.DBus.Error.ServiceUnknown: org.freedesktop.DBus.Error.ServiceUnknown
** Message: 12:15:27.609: gradio-app.vala:135: Registered search provider service.
** Message: 12:15:28.716: gradio-library.vala:75: Open database...
** Message: 12:15:28.717: gradio-library.vala:76: /home/pi/.var/app/de.haeckerfelix.gradio/data/gradio/gradio.db
** Message: 12:15:28.719: gradio-library.vala:46: Successfully opened database! Reading database data...
** Message: 12:15:29.816: gradio-main-window.vala:271: Changed page mode to "library"

(The Library screen is opened, then, when selecting a station:

** Message: 12:18:31.188: gradio-audio-player.vala:83: Connecting to: http://stream.srg-ssr.ch/m/rsc_de/mp3_128
AL lib: (EE) ALCplaybackAlsa_open: Could not open playback device 'default': Connection refused
AL lib: (EE) ALCplaybackAlsa_open: Could not open playback device 'default': Connection refused

Steps to Reproduce

see above

Detailed Description

also see above

sound is OK with VLC and omxplayer!

Thanks and regards,

hugootto