But: there is sound with headset. If somebody call me, we can hear each other too - without headset. But music, youtube, rings and so others are “lost”… Last time I had made the syncronisation and tried out the “HelloCodi” tutorial program. Never ever had problem with this phone.
What ‘syncronisation’ are you talking about Attila?
Did you check the phone’s volume control? Is the volume turned up?
Check the device Settings > Sound > System … are the sounds turned on or off
also check Volume for Notifications, Media and System and their volume settings.
Running HelloCodi does not change any of you phone’s Settings. All it uses is 
It is unlikely coding/running the app affected anything in your phone.
Regards,
Steve