After install volumeicon-alsa, run volumeicon in terminal, next click on it in lxpanel, open Preferences, click on any checkbox in keyboard events, not working? return to terminal,

Unable to find simple control 'Master'

find your volumeicon configuration file (~/.config/volumeicon/volumeicon) and check if [Alsa] section is declared, for example:

[Alsa]
card=hw:1

Open your openbox configuration file (~/.config/openbox/lubuntu-rc.xml), and search this lines:

 <!-- Keybinding for Volume management -->
    <keybind key="XF86AudioRaiseVolume">
      <action name="Execute">
        <command>amixer -q sset Master 10%+ unmute</command>
      </action>
    </keybind>
    <keybind key="XF86AudioLowerVolume">
      <action name="Execute">
        <command>amixer -q sset Master 10%- unmute</command>
      </action>
    </keybind>
    <keybind key="XF86AudioMute">
      <action name="Execute">
        <command>amixer -q sset Master toggle</command>
      </action>
    </keybind>

run in terminal

amixer -q sset Master 10%- unmute

what you see?

amixer: Unable to find simple control 'Master',0
admin@PU551:~$ cat /proc/asound/cards
 0 [HDMI           ]: HDA-Intel - HDA Intel HDMI
                      HDA Intel HDMI at 0xf7f1c000 irq 52
 1 [PCH            ]: HDA-Intel - HDA Intel PCH
                      HDA Intel PCH at 0xf7f18000 irq 51

So, we have to tell the system, which sound card we can use,

amixer -c 1 Master +10

there are II methods:
(I) replace commands on lubuntu-rc.xml,
(II) create custom file with alsa configuration

let’s try the second method:

touch ~/.asoundrc

open in editor and paste the code below:

pcm.!default {
	type hw
	card 1
}
ctl.!default {
	type hw
	card 1
}

or another solution (not tested)

defaults.pcm.!card 1
defaults.pcm.!device 0

source:
https://bbs.archlinux.org/viewtopic.php?id=200806

admin@PU551:~$ sudo alsa force-reload
Unloading ALSA sound driver modules: snd-soc-sst-acpi snd-seq-midi snd-seq-midi-event snd-seq snd-rawmidi snd-seq-device snd-soc-rt5640 snd-soc-rl6231 snd-soc-core snd-compress snd-pcm-dmaengine snd-hda-codec-hdmi snd-hda-codec-conexant snd-hda-codec-generic snd-hda-intel snd-hda-codec snd-hda-core snd-hwdep snd-pcm snd-timer (failed: modules still loaded: snd-hda-codec-hdmi snd-hda-codec-conexant snd-hda-codec-generic snd-hda-intel snd-hda-codec snd-hda-core snd-hwdep snd-pcm snd-timer).
Loading ALSA sound driver modules: snd-soc-sst-acpi snd-seq-midi snd-seq-midi-event snd-seq snd-rawmidi snd-seq-device snd-soc-rt5640 snd-soc-rl6231 snd-soc-core snd-compress snd-pcm-dmaengine snd-hda-codec-hdmi snd-hda-codec-conexant snd-hda-codec-generic snd-hda-intel snd-hda-codec snd-hda-core snd-hwdep snd-pcm snd-timer.

fixed? enjoy!

src:
http://www.alsa-project.org/main/index.php/Asoundrc

there is another method to unlock:
https://itsfoss.com/fix-sound-ubuntu-1304-quick-tip/

more examples for amixer usage:

amixer -c 1 sset Master 5+
amixer sset Master 10%-
amixer sset Master mute
amixer sset Master unmute
amixer sset Master toggle