umount channels

amixer set Master 75 unmute
amixer set PCM 75 unmute
amixer set IEC958 100 unmute

# save settings
alsactl store

# test
aplay /usr/share/sounds/alsa/Noise.wav

Backlist HDMI sound modules from display

echo "blacklist snd_hda_codec_hdmi" | sudo tee -a /etc/modprobe.d/blacklist.conf

Links
http://wiki.archlinux.org/index.php/ALSA_Einrichten