Now the sound is very bad, so I installed the Pifiberry DAC +.

Only change:
under /boot/config.txt the line

dtparam = audio = on

commented out and
dtoverlay = hifiberry-dacplus
inserted.

Now a file
/etc/asound.conf
created with the following content:

pcm.! default {
type hw card 0
}
ctl! default
type hw card 0
}

Then a restart. now the sound comes over the pifiberry. Everything works fine with the Spotify module. But MMM-Screencast is no longer playing Youtube videos: ((

Does one have an idea why it could be? or an alternative to stream Youtube videos ???