The computer has an HDA Intel chip (Realtek ALC887) and I couldn't open alsamixer (this error was displayed: "load hw:0 error: Invalid argument"), although the sound seemed to be working in various applications (no error there) except I couldn't hear anything.

The fix:

Edit /etc/modprobe.d/alsa-base.conf:
sudo gedit /etc/modprobe.d/alsa-base.conf
And add the following line:

options snd-hda-intel model=generic

Save, reboot (a logout might be enough too) and the sound should work. If this generic fix doesn't work for you, see THIS (http://ubuntuforums.org/showthread.php?t=314383) post.

无需重启，运行下面命令即可：

sudo alsa force-reload