I had a hell of a time getting my volume keys to work in i3 4.12 + Ubuntu 16.04
Mute toggle worked, but the unmute + volume raise/lower would only unmute master, and not "speaker".
The following worked to ensure that raising or lowering the volume also unmutes master AND speaker.
bindsym XF86AudioRaiseVolume exec amixer -q -D pulse -c 1 -q sset Master 2dB+ unmute && amixer -D pulse set Master unmute
bindsym XF86AudioLowerVolume exec amixer -q -D pulse -c 1 -q sset Master 2dB- unmute && amixer -D pulse set Master unmute
bindsym XF86AudioMute exec amixer -q -D pulse sset Master toggle