Lubuntu 18.04 - sound applet on lxpanel not revealing volume level when mouse hovers over it

[ardour]

1) The release of Ubuntu you are using, via 'lsb_release -rd' or System -> About Ubuntu

~$ lsb_release -rd
Description: Ubuntu 18.04.1 LTS
Release: 18.04

2) The version of the package you are using, via 'apt-cache policy pkgname' or by checking in Software Center

~$ apt-cache policy lxpanel
lxpanel:
  Installed: 0.9.3-1ubuntu3
  Candidate: 0.9.3-1ubuntu3

3) What you expected to happen

Hovering my mouse over the sound applet should reveal the volume level, as it did in Lubuntu 16.04.

4) What happened instead

Hovering my mouse over the sound applet does not reveal volume level.

[ardour (ardouronerous)]

In this image, I'm hovering my mouse over the sound applet no volume level indicator is popping out, unlike in Lubuntu 16.04.