doc: https://support.hifiberry.com/hc/en-us/articles/205377651-Configuring-Linux-4-x-or-higher
in /boot/config.txt:
dtparam=audio=on
dtoverlay=hifiberry-dacplus
create /etc/asound.conf:
pcm.!default {
type hw card 0
}
ctl.!default {
type hw card 0
}