PulseAudio controls underlying ALSA-level volume controls. To change the ALSA-level volume controls directly, you can do the following:
Open a terminal. (The quickest way is the Ctrl-Alt-T shortcut.)
Enter "alsamixer" and press the Enter key.
You will now see a user interface. In this user interface, you can do the following:
Select your correct sound card using F6 and select F5 to see recording controls as well
Move around with left and right arrow keys.
Increase and decrease volume with up and down arrow keys.
Increase and decrease volume for left/right channel individually with "Q", "E", "Z", and "C" keys.
Mute/Unmute with the "M" key. An "MM" means muted, and "OO" means unmuted. Note that a bar can be 100% full but still be muted, so do check for this.
Exit from alsamixer with the Esc key.
A caveat here: When you mute or unmute something, pulseaudio might pick it up and mute and unmute other controls, as well as PulseAudio's main mute.
If you discover that you need to do something special here to make your sound work, it is likely a bug. Please report a bug using the "ubuntu-bug audio" command. Thanks!
For more information, you can see Wikipedia's alsamixer page.