Are you setting output = i2s
or output = hifiberry_dacplus
? You must set it to output = hifiberry_dacplus
as per the comments in settings.ini. If you are setting it correctly then please try the very latest version (http://www.woutervanwijk.nl/pimusicbox/musicbox0.5.1rc2.zip) and provide the output of
lsmod
cat /sys/class/sound/card*/device/modalias
aplay -l
cat /etc/asound.conf
cat /etc/modules
cat /etc/modprobe.d/blacklist.conf