Hi there....
The easiest way really is to install i3blocks (should be in the repos)
After install via synaptic or commandline...
The edit your i3 config file. At the bottom you will find this.
bar {
status_command i3status
}
Change it to this
bar {
status_command i3blocks
}
Then restart i3 (Mod-Shift-R) - your status line look will change slightly (which you can fully customize of course) and you will see a "VOL" entry typically that you can then use.
i3blocks supports clickable events, etc. On my systems I put my mouse over the "VOL 50%" area, and scroll the mouse wheel and volume goes up and down with no additional config or changes needed. You might need to on your system, but thats one option.
Additionally on i3status you can make some edits as well to make it work, but I like the easy path
