Is there a sample source code of App or any SDK or anything how to build app that receive music stream from server?
You can't stream music from a server, but you can sync music to be played offline.
Official Garmin sample: [https://github.com/garmin/connectiq-apps/tree/master/audio-provider]
SubMusic repo: [https://github.com/memen45/SubMusic]
> Synchronize playlists from your own music server: Nextcloud - Subsonic - Ampache - Airsonic - Plex
CIQ Music Provider FAQ: [https://developer.garmin.com/connect-iq/connect-iq-faq/how-do-i-create-an-audio-content-provider/]
