Which sound card are you using?
lspci | grep Audio
00:1b.0 Audio device: Intel Corporation 6 Series/C200 Series Chipset Family High Definition Audio Controller (rev 05)
If so, I think you have a common problem with many new laptops: you are lacking the 2.1 surround mode enabled in your audio driver. Here a solution that worked for me: intel audio fix
here are three things to do:
echo 'options snd-hda-intel model=asus-mode4' | sudo tee -a /etc/modprobe.d/alsa-base.conf
sudo gedit /usr/share/pulseaudio/alsa-mixer/profile-sets/extra-hdmi.conf
device-strings = surround40:%f
channel-map = front-left,front-right,lfe,lfe
priority = 7
direction = out
Note: On Asus N76VB running on the newer kernel
3.8.0-30, change the last line to be this instead:
direction = output
sudo gedit /etc/pulse/daemon.conf
enable-lfe-remixing = yes
You should now be able to see your "analog surround 2.1" among your possible Hardware settings in the sound menu.
"Analog Surround 4.0" is also working but changes the loudness from Front to Rear.
Additionally you can install the GNOME alsa mixer to boot your basses.