Comment 7 for bug 217789

I have the same soundcard you have (Realtek ALC888) and had the same problem (low sound) before (Mainboard MSI P35 Neo-F)

I added to my /etc/modprobe.d/alsa-base the following line:
options snd-hda-intel model=6stack-dig

I read the Alsa Config File and this one seemed to be the correct one for me (6stacks on the back, 2 front and a not used digital out)

Perhaps it is worth a try for you.

Before trying:
dmesg |grep -i alc
should report something about unknown HDA model, asking bios iirc.

you can control with alsamixer -c0 or the gnome mixer applet the headphone (enable / disable). Please make sure to get a reasonable loud output for both "green outputs" Front and Back - one slider for both - while adjusting "front" to max.

Other model options for your hardware:
 ALC883/888
          3stack-dig 3-jack with SPDIF I/O
          6stack-dig 6-jack digital with SPDIF I/O
          3stack-6ch 3-jack 6-channel
          3stack-6ch-dig 3-jack 6-channel with SPDIF I/O
          6stack-dig-demo 6-jack digital for Intel demo board
          acer Acer laptops (Travelmate 3012WTMi, Aspire 5600, etc)
          acer-aspire Acer Aspire 9810
          medion Medion Laptops
          medion-md2 Medion MD2
          targa-dig Targa/MSI
          targa-2ch-dig Targs/MSI with 2-channel
          laptop-eapd 3-jack with SPDIF I/O and EAPD (Clevo M540JE, M550JE)
          lenovo-101e Lenovo 101E
          lenovo-nb0763 Lenovo NB0763
          lenovo-ms7195-dig Lenovo MS7195
          haier-w66 Haier W66
          3stack-hp HP machines with 3stack (Lucknow, Samba boards)
          6stack-dell Dell machines with 6stack (Inspiron 530)
          mitac Mitac 8252D
          clevo-m720 Clevo M720 laptop series
          fujitsu-pi2515 Fujitsu AMILO Pi2515
          auto auto-config reading BIOS (default)

Source:
zless /usr/share/doc/alsa-base/driver/ALSA-Configuration.txt.gz

Please report back if this makes the sound louder for you!