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
lsmodcat /sys/class/sound/card*/device/modaliasaplay -lcat /etc/asound.confcat /etc/modulescat /etc/modprobe.d/blacklist.conf